2004 AMC 8 Solutions

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

1.

On a map, a 1212-centimeter length represents 7272 kilometers. How many kilometers does a 1717-centimeter length represent?

66

102102

204204

864864

12241224

Concepts:ratio and proportion
Difficulty rating: 370
Small Hint:

Find how many kilometers 11 centimeter represents.

Big Hint:

Multiply that scale by 17.17.

Solution:

Note that 11 cm represents 72÷12=672 \div 12 = 6 kilometers. This means that 1717 cm represents 617=1026 \cdot 17 = 102 kilometers.

Thus, B is the correct answer.

2.

How many different four-digit numbers can be formed by rearranging the four digits in 2004?2004?

44

66

1616

2424

8181

Difficulty rating: 450
Small Hint:

A four-digit number cannot start with 0.0.

Big Hint:

Choose which nonzero digit comes first, then place the other nonzero digit.

Solution:

Note that there are 22 non-zero digits that could be the thousands digit.

After choosing that, we need to arrange the other 33 digits. There are 33 spots for the other non-zero digit.

This gives us 23=62 \cdot 3 = 6 possible numbers.

Thus, B is the correct answer.

3.

Twelve friends met for dinner at Oscar’s Overstuffed Oyster House, and each ordered one meal. The portions were so large, there was enough food for 1818 people. If they shared, how many meals should they have ordered to have just enough food for the 1212 of them?

88

99

1010

1515

1818

Difficulty rating: 560
Small Hint:

The 1212 meals made enough food for 1818 people.

Big Hint:

Find how many meals are needed per person, then scale to 1212 people.

Solution:

Note that 1212 meals feed 1818 people. This means that one meal feeds 1812=32\dfrac{18}{12} = \dfrac{3}{2} people.

This means that they need 12÷32=812 \div \dfrac{3}{2} = 8 meals for 1212 people.

Thus, A is the correct answer.

4.

Ms. Hamilton’s eighth-grade class wants to participate in the annual three-person-team basketball tournament. Lance, Sally, Joy, and Fred are chosen for the team. In how many ways can the three starters be chosen?

22

44

66

88

1010

Difficulty rating: 660
Small Hint:

Choosing the three starters is the same as choosing who sits out.

Big Hint:

There are 44 possible players who could be the alternate.

Solution:

If there are 33 starters, then one person must not be starting. Choosing the person who doesn’t start determines the starters.

There are 44 choices for the person who doesn’t start.

Thus, B is the correct answer.

5.

Ms. Hamilton’s eighth-grade class wants to participate in the annual three-person-team basketball tournament. The losing team of each game is eliminated from the tournament. If sixteen teams compete, how many games will be played to determine the winner?

44

77

88

1515

1616

Difficulty rating: 730
Small Hint:

Each game eliminates exactly one team.

Big Hint:

The tournament ends when all but one team have been eliminated.

Solution:

Note that after every game, one team gets eliminated. For there to be one team remaining, 1515 teams must have been eliminated.

This means that 1515 games had to have been played.

Thus, D is the correct answer.

6.

After Sally takes 2020 shots, she has made 55%55\% of her shots. After she takes 55 more shots, she raises her percentage to 56%.56\%. How many of the last 55 shots did she make?

11

22

33

44

55

Difficulty rating: 870
Small Hint:

Find how many shots Sally had made after 2020 shots.

Big Hint:

Find how many total made shots she had after 2525 shots.

Solution:

Sally made 20×0.55=1120 \times 0.55 = 11 of her first 2020 shots. Then we get that 11+x25=0.56, \dfrac{11 + x}{25} = 0.56, which tells us that 11+x=14 11 + x = 14 and x=3.x = 3.

Thus, C is the correct answer.

7.

An athlete’s target heart rate, in beats per minute, is 80%80\% of the theoretical maximum heart rate. The maximum heart rate is found by subtracting the athlete’s age, in years, from 220.220. To the nearest whole number, what is the target heart rate of an athlete who is 2626 years old?

134134

155155

176176

194194

243243

Difficulty rating: 900
Small Hint:

First compute 22026.220-26.

Big Hint:

Take 80%80\% of that maximum heart rate and round.

Solution:

The maximum heart rate for this athlete would be 22026=194.220 - 26 = 194. Then the target heart rate would be 194×0.8155.194 \times 0.8 \approx 155.

Thus, B is the correct answer.

8.

Find the number of two-digit positive integers whose digits total 7.7.

66

77

88

99

1010

Difficulty rating: 930
Small Hint:

The tens digit cannot be 0.0.

Big Hint:

Once the tens digit is chosen, the ones digit is forced.

Solution:

Note that the tens digit can range from 11 to 7,7, and this digit determines the units digit.

Therefore, there are 77 numbers.

Thus, B is the correct answer.

9.

The average of the five numbers in a list is 54.54. The average of the first two numbers is 48.48. What is the average of the last three numbers?

5555

5656

5757

5858

5959

Concepts:mean
Difficulty rating: 1020
Small Hint:

Turn each average into a sum.

Big Hint:

Subtract the sum of the first two numbers from the sum of all five.

Solution:

The sum of all 55 numbers is 545=270.54 \cdot 5 = 270. The sum of the first 22 numbers is 482=96.48 \cdot 2 = 96.

The sum of the last 33 numbers is 27096=174.270 - 96 = 174. The average is therefore 174÷3=58.174 \div 3 = 58.

Thus, D is the correct answer.

10.

Handy Aaron helped a neighbor 1141 \frac{1}{4} hours on Monday, 5050 minutes on Tuesday, from 8:208:20 to 10:4510:45 on Wednesday morning, and a half-hour on Friday. He is paid $3\$3 per hour. How much did he earn for the week?

$8\$8

$9\$9

$10\$10

$12\$12

$15\$15

Difficulty rating: 1000
Small Hint:

Convert all the work times to minutes.

Big Hint:

After finding total minutes, convert to hours before multiplying by $3.\$3.

Solution:

Aaron worked 7575 minutes on Monday, 5050 minutes on Tuesday, 145145 minutes on Wednesday, and 3030 minutes on Friday.

The total is 75+50+145+30=30075+50+145+30=300 minutes, or 55 hours.

At $3\$3 per hour, he earned 53=155\cdot3=15 dollars.

Thus, E is the correct answer.

11.

The numbers 2,-2, 4,4, 6,6, 99 and 1212 are rearranged according to these rules:

1.1. The largest isn’t first, but it is in one of the first three places.

2.2. The smallest isn’t last, but it is in one of the last three places.

3.3. The median isn’t first or last.

What is the average of the first and last numbers?

3.53.5

55

6.56.5

7.57.5

88

Difficulty rating: 1060
Small Hint:

Use the rules to locate the largest, smallest, and median numbers.

Big Hint:

The first and last positions are filled by the two remaining numbers.

Solution:

Note that the largest, smallest, and median numbers cannot be the first or last number.

This means that the first and last numbers are 44 and 99 in some order. Their average is (4+9)÷2=13÷2=6.5. (4 + 9) \div 2 = 13 \div 2 = 6.5.

Thus, C is the correct answer.

12.

Niki usually leaves her cell phone on. If her cell phone is on but she is not actually using it, the battery will last for 2424 hours. If she is using it constantly, the battery will last for only 33 hours. Since the last recharge, her phone has been on 99 hours, and during that time she has used it for 6060 minutes. If she doesn’t talk any more but leaves the phone on, how many more hours will the battery last?

77

88

1111

1414

1515

Concepts:ratefraction
Difficulty rating: 1170
Small Hint:

One hour of talking uses as much battery as several idle hours.

Big Hint:

The phone was idle for 88 hours and used for 11 hour.

Solution:

When not in use, her cell phone uses up 124\frac{1}{24} of its battery per hour. When it is in use, it uses up 13\frac{1}{3} of its battery per hour.

Niki’s phone has been on for 99 hours, with 88 of those hours being idle and 11 hour being used to talk on the phone.

This means that the phone has used up 23\frac{2}{3} of its battery. In order to drain the remaining 13\frac{1}{3} of the battery, the phone can last for 88 more hours without being used.

Thus, B is the correct answer.

13.

Amy, Bill and Celine are friends with different ages. Exactly one of the following statements is true.

I.\mathrm{I}. Bill is the oldest.

II.\mathrm{II}. Amy is not the oldest.

III.\mathrm{III}. Celine is not the youngest.

Rank the friends from the oldest to youngest.

Bill, Amy, Celine

Amy, Bill, Celine

Celine, Amy, Bill

Celine, Bill, Amy

Amy, Celine, Bill

Difficulty rating: 1190
Small Hint:

If Bill were oldest, check how many statements would be true.

Big Hint:

After ruling out the oldest person, use the fact that exactly one statement is true.

Solution:

If Bill were oldest, then statements I\mathrm{I} and II\mathrm{II} would both be true, so Bill is not oldest.

If Celine were oldest, then statements II\mathrm{II} and III\mathrm{III} would both be true, so Celine is not oldest.

Therefore Amy is oldest. Statements I\mathrm{I} and II\mathrm{II} are false, so statement III\mathrm{III} must be the single true statement. Thus Celine is not youngest, leaving Bill youngest.

The order is Amy, Celine, Bill.

Thus, E is the correct answer.

14.

What is the area enclosed by the geoboard quadrilateral below?

1515

181218\frac{1}{2}

221222\frac{1}{2}

2727

4141

Difficulty rating: 1190
Small Hint:

Put the quadrilateral inside an easy surrounding square.

Big Hint:

Subtract the outside rectangles and right triangles from the surrounding square.

Solution:

Place the quadrilateral inside the surrounding 10×1010\times10 square, whose area is 100.100.

The five outside pieces have areas 15,15, 1267=21,\frac12\cdot6\cdot7=21, 1213=32,\frac12\cdot1\cdot3=\frac32, 1245=10,\frac12\cdot4\cdot5=10, and 12610=30.\frac12\cdot6\cdot10=30.

The outside area is 15+21+32+10+30=7712,15+21+\frac32+10+30=77\frac12, so the quadrilateral area is 1007712=2212.100-77\frac12=22\frac12.

Thus, C is the correct answer.

15.

Thirteen shaded and six unshaded hexagonal tiles were used to create the figure below. If a new figure is created by attaching a border of unshaded tiles with the same size and shape as the others, what will be the difference between the total number of unshaded tiles and the total number of shaded tiles in the new figure?

55

77

1111

1212

1818

Difficulty rating: 1220
Small Hint:

The original figure has one center tile, a ring of 6,6, and a ring of 12.12.

Big Hint:

The next hexagonal border has 1818 tiles.

Solution:

The original figure has 1313 shaded tiles and 66 unshaded tiles.

The new border is the next hexagonal ring around the figure, which has 1818 unshaded tiles.

The new figure has 6+18=246+18=24 unshaded tiles and 1313 shaded tiles, so the difference is 2413=11.24-13=11.

Thus, C is the correct answer.

16.

Two 600600 mL pitchers contain orange juice. One pitcher is 13\frac 13 full and the other pitcher is 25\frac 25 full. Water is added to fill each pitcher completely, then both pitchers are poured into one large container. What fraction of the mixture in the large container is orange juice?

18\dfrac{1}{8}

316\dfrac{3}{16}

1130\dfrac{11}{30}

1119\dfrac{11}{19}

1115\dfrac{11}{15}

Difficulty rating: 1160
Small Hint:

Compute the orange juice in each pitcher before water is added.

Big Hint:

After filling both pitchers, the total mixture volume is 12001200 mL.

Solution:

The first pitcher contains 60013=200600 \cdot \dfrac{1}{3} = 200 mL of orange juice. The second one has 60025=240600 \cdot \dfrac{2}{5} = 240 mL.

The large container then has 200+240=440200 + 240 = 440 mL of orange juice. The total amount of mixture is 2600=12002 \cdot 600 = 1200 mL.

Then the fraction of orange juice is 4401200=1130.\dfrac{440}{1200} = \dfrac{11}{30}.

Thus, C is the correct answer.

17.

Three friends have a total of 66 identical pencils, and each one has at least one pencil. In how many ways can this happen?

11

33

66

1010

1212

Difficulty rating: 1220
Small Hint:

Give each friend one pencil first.

Big Hint:

Count the ways to distribute the remaining 33 pencils among the 33 friends.

Solution:

First give each friend one pencil. Then 33 pencils remain to distribute among the 33 friends.

If one friend gets all 33 remaining pencils, there are 33 ways. If the remaining pencils split as 22 and 1,1, there are 32=63\cdot2=6 ways. If each friend gets one more, there is 11 way.

The total number of ways is 3+6+1=10.3+6+1=10.

Thus, D is the correct answer.

18.

Five friends compete in a dart-throwing contest. Each one has two darts to throw at the same circular target, and each individual’s score is the sum of the scores in the target regions that are hit. The scores for the target regions are the whole numbers 11 through 10.10. Each throw hits the target in a region with a different value. The scores are: Alice 1616 points, Ben 44 points, Cindy 77 points, Dave 1111 points, and Ellen 1717 points. Who hits the region worth 66 points?

Alice

Ben

Cindy

Dave

Ellen

Difficulty rating: 1400
Small Hint:

Ben’s score of 44 has only one possible pair of distinct positive scores.

Big Hint:

Use each forced pair to eliminate numbers from the remaining scores.

Solution:

The only way to get Ben’s score of 44 is with a 11 and 33 since he can’t hit 22 twice.

Cindy can achieve her score with 1+6,2+5,3+4. 1 + 6,\quad 2 + 5,\quad 3 + 4. Ben already hit 11 and 3,3, so Cindy must have hit 22 and 5.5.

Similarly, Dave must have hit 44 and 7.7. Finally, since 77 is already used, Alice is forced to have hit 66 and 1010 with Ellen hitting 88 and 9.9.

Thus, A is the correct answer.

19.

A whole number larger than 22 leaves a remainder of 22 when divided by each of the numbers 3,3, 4,4, 5,5, and 6.6. The smallest such number lies between which two numbers?

4040 and 4949

6060 and 7979

100100 and 129129

210210 and 249249

320320 and 369369

Difficulty rating: 1220
Small Hint:

If the number leaves remainder 2,2, subtract 2.2.

Big Hint:

The result must be divisible by 3,3, 4,4, 5,5, and 6.6.

Solution:

Let xx be the number. Then x2x - 2 is divisible by 3,3, 4,4, 5,5, and 6.6.

The least common multiple of these numbers is 60,60, which makes xx 62.62.

Thus, B is the correct answer.

20.

Two-thirds of the people in a room are seated in three-fourths of the chairs. The rest of the people are standing. If there are 66 empty chairs, how many people are in the room?

1212

1818

2424

2727

3636

Difficulty rating: 1230
Small Hint:

The empty chairs are the remaining one-fourth of the chairs.

Big Hint:

The seated people are two-thirds of all the people.

Solution:

Since 34\frac{3}{4} of the chairs are occupied, the 66 empty chairs are 14\frac{1}{4} of all the chairs. Thus there are 2424 chairs.

The number of seated people is 34\frac{3}{4} of 24,24, which is 18.18.

Those 1818 seated people are 23\frac{2}{3} of all the people, so the total number of people is 18÷23=27.18\div\frac23=27.

Thus, D is the correct answer.

21.

Spinners AA and BB are spun. On each spinner, the arrow is equally likely to land on each number. What is the probability that the product of the two spinners’ numbers is even?

14\dfrac{1}{4}

13\dfrac{1}{3}

12\dfrac{1}{2}

23\dfrac{2}{3}

34\dfrac{3}{4}

Difficulty rating: 1290
Small Hint:

It is easier to count the complement: an odd product.

Big Hint:

An odd product requires both spinner results to be odd.

Solution:

For the product to be even, at least one of the spinners must land on an even number.

We can use complementary counting and calculate the probability of both spinners landing on odds.

This happens with a probability of 1223=13. \dfrac{1}{2} \cdot \dfrac{2}{3} = \dfrac{1}{3}.

Then the probability of landing on at least one even is 113=23.1 - \dfrac{1}{3} = \dfrac{2}{3}.

Thus, D is the correct answer.

22.

At a party there are only single women and married men with their wives. The probability that a randomly selected woman is single is 25.\dfrac{2}{5}. What fraction of the people in the room are married men?

13\dfrac{1}{3}

38\dfrac{3}{8}

25\dfrac{2}{5}

512\dfrac{5}{12}

35\dfrac{3}{5}

Difficulty rating: 1450
Small Hint:

Use a convenient number of women matching the ratio 25.\frac{2}{5}.

Big Hint:

Married women and married men come in equal numbers.

Solution:

For a convenient ratio model, let there be 55 women in the room. Then there are 525=25 \cdot \dfrac{2}{5} = 2 single women.

This means that there are 52=35 - 2 = 3 married women, which is also the number of married men.

There are a total of 5+3=85 + 3 = 8 people in the room. The fraction of married men is 38.\dfrac{3}{8}.

Thus, B is the correct answer.

23.

Tess runs counterclockwise around rectangular block JKLM.JKLM. She lives at corner J.J. Which graph could represent her straight-line distance from home?

Difficulty rating: 1450
Small Hint:

Track the distance from corner JJ at each side of the rectangle.

Big Hint:

The distance is greatest at the opposite corner L.L.

Solution:

As Tess runs from JJ to K,K, her distance from home increases.

From KK to L,L, her distance continues to increase, but with a different shape, reaching its maximum at the opposite corner L.L.

From LL to MM and then from MM back to J,J, her distance decreases back to 0,0, again changing behavior at M.M.

Graph DD is the only graph with this increase, changed-rate increase, changed-rate decrease, and final decrease to 0.0.

Thus, D is the correct answer.

24.

In the figure, ABCDABCD is a rectangle and EFGHEFGH is a parallelogram. Using the measurements given in the figure, what is the length dd of the segment that is perpendicular to HE\overline{HE} and FG?\overline{FG}?

6.86.8

7.17.1

7.67.6

7.87.8

8.18.1

Difficulty rating: 1540
Small Hint:

Find the area of the rectangle and subtract the four corner triangles.

Big Hint:

Use the Pythagorean theorem to get the parallelogram base HE.HE.

Solution:

The rectangle has side lengths 1010 and 8,8, so its area is 80.80.

The four corner triangles have total area 21234+21265=12+30=42. \begin{aligned} &2\cdot\frac12\cdot3\cdot4 \\ &\quad{}+2\cdot\frac12\cdot6\cdot5 \\ &=12+30=42. \end{aligned} Thus parallelogram EFGHEFGH has area 8042=38.80-42=38.

Segment HEHE has length 55 by the 3453-4-5 right triangle. Since the parallelogram area is HEd,HE\cdot d, we have 5d=38,5d=38, so d=7.6.d=7.6.

Thus, C is the correct answer.

25.

Two 4×44 \times 4 squares intersect at right angles, bisecting their intersecting sides, as shown. The circle’s diameter is the segment between the two points of intersection. What is the area of the shaded region created by removing the circle from the squares?

164π16-4\pi

162π16-2\pi

284π28-4\pi

282π28-2\pi

322π32-2\pi

Difficulty rating: 1580
Small Hint:

Find the area covered by the union of the two squares.

Big Hint:

The circle diameter is the diagonal of the 2×22\times2 overlap square.

Solution:

The two 4×44\times4 squares have total area 32,32, but their overlap is a 2×22\times2 square with area 4.4. Thus the area covered by the union of the two squares is 324=28.32-4=28.

The circle’s diameter is the diagonal of that 2×22\times2 overlap square, so the diameter is 222\sqrt2 and the radius is 2.\sqrt2.

The circle area is π(2)2=2π,\pi(\sqrt2)^2=2\pi, so the shaded area is 282π.28-2\pi.

Thus, D is the correct answer.