2022 AMC 8 Solutions

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All problems are used with official legal permission of the Mathematical Association of America (MAA).

1.

The Math Team designed a logo shaped like a multiplication symbol, shown below on a grid of 11-inch squares. What is the area of the logo in square inches?

1010

1212

1313

1414

1515

Concepts:area decomposition
Difficulty rating: 770
Small Hint:

Break the logo into congruent tilted squares

Big Hint:

Each tilted square is half of a 22 by 22 grid square

Solution:

The logo is made from 55 congruent tilted squares. Each tilted square has both diagonals of length 22, so its area is 222=2\frac{2\cdot2}{2}=2 square inches.

The total area is 52=105\cdot2=10.

Thus, the correct answer is A.

2.

Consider these two operations: ab=a2b2ab=(ab)2\begin{align*} a\,\blacklozenge\,b &= a^2 - b^2 \\ a \star b &= (a - b)^2 \end{align*}

Compute the value:

(53)6 (5\,\blacklozenge\,3) \star 6

20-20

44

1616

100100

220220

Difficulty rating: 560
Small Hint:

Evaluate 535\,\blacklozenge\,3 first

Big Hint:

Substitute the value from the first operation into 6\star 6

Solution:

Using the definitions, (53)6=(5232)6=166=(166)2=100\begin{align*} (5\,\blacklozenge\,3) \star 6 &= (5^2-3^2) \star 6 \\ &=16 \star 6 \\ &=(16-6)^2\\ &= 100\end{align*}

Thus, the correct answer is D.

3.

When three positive integers a,a, b,b, and cc are multiplied together, their product is 100.100. Suppose a<b<c.a < b < c. In how many ways can the numbers be chosen?

00

11

22

33

44

Difficulty rating: 1190
Small Hint:

The smallest number must be a factor of 100100

Big Hint:

Try possible values of aa with a3<100a^3<100

Solution:

Since a<b<ca< b< c and abc=100abc=100, we have a3<100a^3<100, so a4a\le4. Also aa must divide 100100, so a{1,2,4}a\in\{1,2,4\}.

If a=1a=1, the pairs (b,c)(b,c) are (2,50),(4,25),(5,20)(2,50),(4,25),(5,20). If a=2a=2, the only possible pair is (5,10)(5,10). If a=4a=4, then bc=25bc=25, and there is no integer bb with 4<b<c4< b< c.

There are 44 possible triples.

Thus, the correct answer is E.

4.

The letter M in the figure below is first reflected over the line qq and then reflected over the line p.p. What is the resulting image?

Difficulty rating: 770
Small Hint:

Reflecting over qq moves the letter across the diagonal line

Big Hint:

After the first reflection, reflect the new image over the horizontal line pp

Solution:

Reflecting over the diagonal line qq changes the orientation of the letter as if the diagonal were a mirror. Reflecting that image over the horizontal line pp then places the final letter below pp, to the right of the vertical line, with the orientation shown in choice E.

Thus, the correct answer is E.

5.

Anna and Bella are celebrating their birthdays together. Five years ago, when Bella turned 66 years old, she received a newborn kitten as a birthday present. Today the sum of the ages of the two children and the kitten is 3030 years. How many years older than Bella is Anna?

11

22

33

44

55

Difficulty rating: 720
Small Hint:

Bella is 55 years older now than when she turned 66

Big Hint:

Find Bella’s current age and the kitten’s current age before finding Anna’s

Solution:

If Bella was 66 five years ago, then she is 1111 right now.

If the kitten was a newborn five years ago, then it is 55 right now.

Since the sum of all 33 ages is 30,30, Anna’s age is 30511=1430-5-11 = 14

Since Anna is 1414 and Bella is 11,11, she is 33 years older than Bella.

Thus, the correct answer is C.

6.

Three positive integers are equally spaced on a number line. The middle number is 1515 and the largest number is 44 times the smallest number. What is the smallest of these three numbers?

44

55

66

77

88

Difficulty rating: 820
Small Hint:

The middle of three equally spaced numbers is their average

Big Hint:

If the smallest is xx, then the largest is 4x4x

Solution:

Let the smallest number be xx. Then the largest is 4x4x. Since the three numbers are equally spaced, the middle number is the average of the smallest and largest: x+4x2=15. \frac{x+4x}{2}=15. Thus 5x=305x=30, so x=6x=6.

Thus, the correct answer is C.

7.

When the World Wide Web first became popular in the 1990s, download speeds reached a maximum of about 5656 kilobits per second. Approximately how many minutes would the download of a 4.24.2-megabyte song have taken at that speed? (Note that there are 80008000 kilobits in a megabyte.)

0.60.6

1010

18001800

72007200

3600036000

Difficulty rating: 960
Small Hint:

Convert megabytes to kilobits first

Big Hint:

4.280004.2\cdot8000 kilobits divided by 5656 kilobits per second gives seconds

Solution:

The song has 4.28000=33,6004.2\cdot8000=33{,}600 kilobits. At 5656 kilobits per second, the download takes 33,60056=600 \frac{33{,}600}{56}=600 seconds, which is 1010 minutes.

Thus, the correct answer is B.

8.

What is the value of:

132435182019212022 \dfrac{1}{3} \cdot \dfrac{2}{4} \cdot \dfrac{3}{5} \cdots \dfrac{18}{20} \cdot \dfrac{19}{21} \cdot \dfrac{20}{22}

1462\displaystyle \dfrac{1}{462}

1231\displaystyle \dfrac{1}{231}

1132\displaystyle \dfrac{1}{132}

2213\displaystyle \dfrac{2}{213}

122\displaystyle \dfrac{1}{22}

Difficulty rating: 1020
Small Hint:

Most numerator and denominator factors cancel

Big Hint:

After cancellation, only 121\cdot2 remains on top

Solution:

Since every integer from 33 to 2020 occurs once as a denominator and once as a numerator, they cancel each other out.

After canceling every number out, we have only 11 and 22 left as numerators and 2121 and 2222 left as denominators.

The remaining fraction is 122122. \dfrac{1 \cdot 2}{21 \cdot 22} . This simplifies to 2462=1231 \dfrac{2}{462} = \dfrac{1}{231}

Thus, the correct answer is B.

9.

A cup of boiling water (212212^\circ F) is placed to cool in a room whose temperature remains constant at 6868^\circ F. Suppose the difference between the water temperature and the room temperature is halved every 55 minutes. What is the water temperature, in degrees Fahrenheit, after 1515 minutes?

7777

8686

9292

9898

104104

Difficulty rating: 900
Small Hint:

Track the difference from room temperature

Big Hint:

In 1515 minutes, the difference is halved 33 times

Solution:

The current difference is 21268=144.212-68 = 144 .

Since we have 1515 minutes while halving every 55 minutes, we halve the difference 33 times.

This means the difference is multiplied by (12)3=18,(\dfrac{1}{2})^3 = \dfrac{1}{8} , so our new difference is 18144=18.\dfrac{1}{8}\cdot144=18. This makes our final temperature 68+18=86.68+18=86.

Thus, the correct answer is B.

10.

One sunny day, Ling decided to take a hike in the mountains. She left her house at 88 a.m., drove at a constant speed of 4545 miles per hour, and arrived at the hiking trail at 1010 a.m. After hiking for 33 hours, Ling drove home at a constant speed of 6060 miles per hour. Which of the following graphs best illustrates the distance between Ling’s car and her house over the course of her trip?

Difficulty rating: 900
Small Hint:

Find the distance to the trail from the first drive

Big Hint:

The return drive is faster, so it takes less time than the outward drive

Solution:

She drives 4545 miles per hour for 22 hours, so the trail is 9090 miles from her house. The graph must rise from 00 to 9090 miles between 88 AM and 1010 AM, then stay flat for the 33-hour hike.

She starts home at 11 PM. Driving 9090 miles at 6060 miles per hour takes 1.51.5 hours, so she gets home at 2:302{:}30 PM. This matches choice E.

Thus, the correct answer is E.

11.

Henry the donkey has a very long piece of pasta. He takes a number of bites of pasta, each time eating 33 inches of pasta from the middle of one piece. In the end, he has 1010 pieces of pasta whose total length is 1717 inches. How long, in inches, was the piece of pasta he started with?

3434

3838

4141

4444

4747

Concepts:invariant
Difficulty rating: 1020
Small Hint:

Each bite increases the number of pasta pieces by 11

Big Hint:

To end with 1010 pieces, Henry made 99 bites

Solution:

Since there are 1010 pieces, there were 99 locations where a bite was made. Since we have 99 bites and 33 inches are removed per bite, a total of 2727 inches were removed.

With 2727 inches removed and 1717 inches remaining, we know we started with 27+17=4427+17 = 44 inches.

Thus, the correct answer is D.

12.

The arrows on the two spinners shown below are spun. Let the number NN equal 1010 times the number on Spinner A, added to the number on Spinner B. What is the probability that NN is a perfect square number?

116\displaystyle \dfrac{1}{16}

18\displaystyle \dfrac{1}{8}

14\displaystyle \dfrac{1}{4}

38\displaystyle \dfrac{3}{8}

12\displaystyle \dfrac{1}{2}

Difficulty rating: 1100
Small Hint:

Spinner A gives the tens digit and spinner B gives the ones digit

Big Hint:

Check which two-digit squares have tens digit 5,6,7,85,6,7,8 and ones digit 1,2,3,41,2,3,4

Solution:

Spinner A gives the tens digit and Spinner B gives the ones digit. There are 44=164\cdot4=16 equally likely two-digit numbers.

The possible perfect squares between 5151 and 8484 are 6464 and 8181, and both can occur. Thus the probability is 216=18. \frac{2}{16}=\frac{1}{8}.

Thus, the correct answer is B.

13.

How many positive integers can fill the blank in the sentence below?

“One positive integer is ___ more than twice another, and the sum of the two numbers is 2828

66

77

88

99

1010

Difficulty rating: 1100
Small Hint:

Let the smaller of the two positive integers be xx

Big Hint:

If the blank is cc, the two numbers can be xx and 2x+c2x+c

Solution:

Let the smaller number be xx, and let the blank be cc. Then the other number is 2x+c2x+c, where both xx and cc are positive integers.

The sum condition gives x+(2x+c)=28x+(2x+c)=28, so c=283xc=28-3x. For cc to be positive, 283x>028-3x>0, so x<283x<\frac{28}{3}.

Thus xx can be any integer from 11 through 99, giving 99 possible values of the blank.

Thus, the correct answer is D.

14.

In how many ways can the letters in BEEKEEPER be rearranged so that two or more E’s do not appear together?

11

44

1212

2424

120120

Difficulty rating: 1190
Small Hint:

There are 55 E’s and 44 other letters

Big Hint:

The four non-E letters must fill the four gaps between the E’s

Solution:

The word has 55 E’s and 44 other letters: B, K, P, and R. To keep no two E’s adjacent, the only possible pattern is E_E_E_E_E, E\_E\_E\_E\_E, with the four non-E letters in the four gaps.

The letters B, K, P, and R can be arranged in those gaps in 4!=244!=24 ways.

Thus, the correct answer is D.

15.

Laszlo went online to shop for black pepper and found thirty different black pepper options varying in weight and price, shown in the scatter plot below. In ounces, what is the weight of the pepper that offers the lowest price per ounce?

11

22

33

44

55

Difficulty rating: 1140
Small Hint:

Compare price divided by weight

Big Hint:

For each weight, only the lowest plotted price can be best

Solution:

For each weight, only the lowest-price point at that weight can give the lowest price per ounce.

At 11 ounce, the lowest price is more than 11 dollar, so the price per ounce is more than 11.

At 22 ounces, the lowest price is 22 dollars, so the price per ounce is 11.

At 33 ounces, the lowest price is about 2.52.5 dollars, so the price per ounce is about 2.53\frac{2.5}{3}, less than 11.

At 44 ounces, the lowest price is close to 3.93.9 dollars, so the price per ounce is close to 3.94\frac{3.9}{4}, still larger than the 33-ounce option.

At 55 ounces, the lowest price is close to 4.54.5 dollars, so the price per ounce is close to 4.55\frac{4.5}{5}, also larger than the 33-ounce option.

The price per ounce is lowest at 33 ounces.

Thus, the correct answer is C.

16.

Four numbers are written in a row. The average of the first two is 21,21, the average of the middle two is 26,26, and the average of the last two is 30.30. What is the average of the first and last of the numbers?

2424

2525

2626

2727

2828

Difficulty rating: 1100
Small Hint:

Convert each average into a sum

Big Hint:

Find a+b+c+da+b+c+d, then subtract b+cb+c

Solution:

Let the numbers be a,b,c,da,b,c,d in order.

Since the average of aa and bb is 21,21, we know their sum is 212=42.21\cdot2 = 42.
Since the average of cc and dd is 30,30, we know their sum is 302=60.30\cdot2 = 60.
Since a+b=42a+b = 42 and c+d=60,c+d = 60 , we know a+b+c+d=42+60=102\begin{align*} a+b+c+d &= 42 + 60\\ &= 102 \end{align*}

Now, with the average of bb and cc being 26,26, we know their sum is 262=52.26\cdot2=52. This means b+c=52.b+c = 52.

Subtracting this result from the sum of all the terms yields a+d=50.a+d =50.
Since a+d2=25,\dfrac{a+d}{2}=25, our answer is 25.25.

Thus, the correct answer is B.

17.

If nn is an even positive integer, the double factorial notation n!!n!! represents the product of all the even integers from 22 to n.n. For example:

8!!=2×4×6×8 8!! = 2 \times 4 \times 6 \times 8

What is the units digit of the following sum?

2!!+4!!+6!!++2018!!+2020!!+2022!! \begin{aligned} &2!! + 4!! + 6!! + \cdots \\ &\quad {}+ 2018!! + 2020!! + 2022!! \end{aligned}

00

22

44

66

88

Difficulty rating: 1190
Small Hint:

Any even double factorial at least 10!!10!! has a factor of 1010

Big Hint:

Only 2!!,4!!,6!!,2!!,4!!,6!!, and 8!!8!! can affect the units digit

Solution:

For every even n10,n\ge10, the product n!!n!! contains a factor of 10,10, so that term has units digit 0.0. Only the first four terms can affect the units digit.

They sum to 2!!+4!!+6!!+8!!=2+8+48+384=442. \begin{gathered} 2!!+4!!+6!!+8!!\\ =2+8+48+384\\ =442. \end{gathered} Therefore the full sum has units digit 2.2.

Thus, the correct answer is B.

18.

The midpoints of the four sides of a rectangle are (3,0),(-3, 0), (2,0),(2, 0), (5,4),(5, 4), and (0,4).(0, 4). What is the area of the rectangle?

2020

2525

4040

5050

8080

Difficulty rating: 1310
Small Hint:

The given midpoints form a parallelogram inside the rectangle

Big Hint:

The midpoint parallelogram has half the area of the original rectangle

Solution:

The four given midpoints form a parallelogram. Its horizontal base has length 2(3)=52-(-3)=5, and its height is 44, so its area is 54=205\cdot4=20.

For any rectangle, the parallelogram formed by joining the side midpoints has half the area of the rectangle. Therefore the rectangle’s area is 220=402\cdot20=40.

Thus, the correct answer is C.

19.

Mr. Ramos gave a test to his class of 2020 students. The dot plot below shows the distribution of test scores.

Later Mr. Ramos discovered that there was a scoring error on one of the questions. He regraded the tests, awarding some of the students 55 extra points, which increased the median test score to 85.85. What is the minimum number of students who received extra points?

(Note that the median test score equals the average of the 22 scores in the middle if the 2020 test scores are arranged in increasing order.)

22

33

44

55

66

Difficulty rating: 1370
Small Hint:

The median is controlled by the 1010th and 1111th scores

Big Hint:

Count how many scores are at least 8585 before and after regrading

Solution:

All scores are multiples of 5,5, and adding 55 preserves that fact. If the 1010th and 1111th scores average 8585 but are not both 85,85, then the 1111th score must be at least 90.90. That would require at least 1010 scores of 9090 or more.

However, even after regrading, only the original scores of 8585 or more can reach 90.90. The plot has only 77 such scores, so the two middle scores must both be 85.85.

The plot initially has 77 scores of at least 85.85. To make the 1010th and 1111th scores both 85,85, there must be at least 1111 scores of at least 85.85. Raising four of the 8080’s to 8585 is both necessary and sufficient.

Thus, the correct answer is C.

20.

The grid below is to be filled with integers in such a way that the sum of the numbers in each row and the sum of the numbers in each column are the same. Four numbers are missing. The number xx in the lower left corner is larger than the other three missing numbers. What is the smallest possible value of x?x?

1-1

55

66

88

99

Difficulty rating: 1430
Small Hint:

The completed row and column sums must match the top row

Big Hint:

Express the other missing entries in terms of xx

Solution:

Adding the numbers in the top row shows that every row and column must have sum 1212.

In the first column, the missing number above xx is 14x14-x. In the bottom row, the missing number to the right of xx is 4x4-x. In the middle row, the remaining missing number is x1x-1.

Since xx is larger than the other missing numbers, we need x>14xx>14-x, x>4xx>4-x, and x>x1x> x-1. The strongest condition is x>7x>7, so the smallest possible integer value is 88.

Thus, the correct answer is D.

21.

Steph scored 1515 baskets out of 2020 attempts in the first half of a game, and 1010 baskets out of 1010 attempts in the second half. Candace took 1212 attempts in the first half and 1818 attempts in the second. In each half, Steph scored a higher percentage of baskets than Candace. Surprisingly they ended with the same overall percentage of baskets scored. How many more baskets did Candace score in the second half than in the first?

77

88

99

1010

1111

Difficulty rating: 1460
Small Hint:

Steph made 2525 baskets in 3030 attempts overall

Big Hint:

Candace must also have made 2525 baskets total

Solution:

Steph made 2525 baskets in 3030 attempts. Candace also took 12+18=3012+18=30 attempts, and their overall percentages were the same, so Candace also made 2525 baskets.

Let ff be Candace’s first-half baskets and ss be her second-half baskets. Then f+s=25f+s=25.

Steph’s first-half percentage was 1520=34\frac{15}{20}=\frac34, so f12<34\frac{f}{12}<\frac34, giving f<9f<9. Steph’s second-half percentage was 11, so s<18s<18.

Thus f8f\le8 and s17s\le17. Since f+s=25f+s=25, the only possibility is f=8f=8 and s=17s=17, so sf=9s-f=9.

Thus, the correct answer is C.

22.

A bus takes 22 minutes to drive from one stop to the next, and waits 11 minute at each stop to let passengers board. Zia takes 55 minutes to walk from one bus stop to the next. As Zia reaches a bus stop, if the bus is at the previous stop or has already left the previous stop, then she will wait for the bus. Otherwise she will start walking toward the next stop. Suppose the bus and Zia start at the same time toward the library, with the bus 33 stops behind. After how many minutes will Zia board the bus?

1717

1919

2020

2121

2323

Difficulty rating: 1510
Small Hint:

Check Zia’s decision only when she reaches a stop

Big Hint:

Compare positions at 55, 1010, and 1515 minutes

Solution:

The bus takes 22 minutes to drive to the next stop and then waits 11 minute, so it moves through one-stop cycles every 33 minutes.

Zia makes a decision only when she reaches a stop, every 55 minutes. Measure stops from the bus’s starting stop. Zia starts at stop 33.

After 55 minutes, Zia is at stop 44, while the bus is waiting at stop 22, so she keeps walking. After 1010 minutes, Zia is at stop 55, while the bus is between stops 33 and 44, so she keeps walking. After 1515 minutes, Zia is at stop 66, while the bus is at stop 55, the previous stop, so she waits.

The bus then takes 22 more minutes to reach her stop, so she boards after 1717 minutes.

Thus, the correct answer is A.

23.

A \bigtriangleup or \bigcirc is placed in each of the nine squares in a 3×33 \times 3 grid. Shown below is a sample configuration with three \bigtriangleup’s in a line.

How many configurations will have three \bigtriangleup’s in a line and three \bigcirc’s in a line?

3939

4242

7878

8484

9696

Difficulty rating: 1670
Small Hint:

Split by how many triangles are in the grid

Big Hint:

With exactly 33 triangles, diagonal triangle lines do not work

Solution:

Let kk be the number of triangles. To have both a triangle line and a circle line, we need 3k63\le k\le6.

If k=3k=3, the three triangles must form a line. A row or column works, because the remaining six circles contain a full circle row or column. A diagonal does not work, because its complement contains no full line of circles. Thus there are 66 configurations for k=3k=3. By symmetry, there are also 66 configurations for k=6k=6.

If k=4k=4, choose the triangle line and then the extra triangle. If the line is a row or column, all 66 possible extra positions work, because one parallel row or column remains all circles. This gives 66=366\cdot6=36 configurations. If the triangle line is a diagonal, no extra position leaves a full circle line. By symmetry, k=5k=5 also gives 3636 configurations.

The total number of configurations is 6+36+36+6=846+36+36+6=84.

Thus, the correct answer is D.

24.

The figure below shows a polygon ABCDEFGH,ABCDEFGH, consisting of rectangles and right triangles. When cut out and folded on the dotted lines, the polygon forms a triangular prism. Suppose that:

AH=EF=8 AH = EF = 8

and

GH=14. GH = 14.

What is the volume of the prism?

112112

128128

192192

240240

288288

Difficulty rating: 1510
Small Hint:

Find the right-triangle base of the prism

Big Hint:

The folded net gives a base with legs 66 and 88, and prism length 88

Solution:

When the net is folded, the matching side lengths give GF=EF=8GF=EF=8. Since GFCBGFCB is a rectangle, BC=8BC=8. The fold identifies ABAB with BCBC, so AB=8AB=8. In rectangle HJBAHJBA, this gives HJ=8HJ=8, and the opposite side BJBJ equals AH=8AH=8.

Since GH=14GH=14, we have GJ=148=6GJ=14-8=6. Thus one triangular base of the prism is right triangle BJGBJG, with area 682=24. \frac{6\cdot8}{2}=24. The prism length is GF=8GF=8, so the volume is 248=19224\cdot8=192.

Thus, the correct answer is C.

25.

A cricket randomly hops between 44 leaves, on each turn hopping to one of the other 33 leaves with equal probability. After 44 hops, what is the probability that the cricket has returned to the leaf where it started?

29\displaystyle \dfrac{2}{9}

1980\displaystyle \dfrac{19}{80}

2081\displaystyle \dfrac{20}{81}

14\displaystyle \dfrac{1}{4}

727\displaystyle \dfrac{7}{27}

Difficulty rating: 1670
Small Hint:

Track only whether the cricket is on the starting leaf

Big Hint:

If the cricket is not at the start, the next hop returns with probability 13\frac13

Solution:

Let pnp_n be the probability that the cricket is on its starting leaf after nn hops. We have p0=1p_0=1.

If the cricket is on the starting leaf, the next hop must leave it. If the cricket is not on the starting leaf, exactly one of the 33 possible hops returns to the start. Therefore pn+1=1pn3. p_{n+1}=\frac{1-p_n}{3}.

Thus p1=0,p2=13,p3=29,p4=1293=727. \begin{gathered} p_1=0,\quad p_2=\frac13,\quad p_3=\frac29,\quad \\ p_4=\frac{1-\frac29}{3}=\frac{7}{27}. \end{gathered}

Thus, the correct answer is E.