1989 AMC 8 Solutions

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

1.

What is the value of

(1+11+21+31+41)+(9+19+29+39+49)? \begin{aligned} &(1 + 11 + 21 + 31 + 41) \\ &\quad {}+ (9 + 19 + 29 + 39 + 49)? \end{aligned}

150150

199199

200200

249249

250250

Concepts:pairing and grouping
Difficulty rating: 560
Small Hint:

Look for pairs of terms that combine into round numbers

Big Hint:

Pairing 11 with 49,49, 1111 with 39,39, and so on, each pair sums to 5050

Solution:

Pair the terms so each pair sums to 5050: 1+49,1 + 49, 11+39,11 + 39, 21+29,21 + 29, 31+19,31 + 19, and 41+9.41 + 9.

There are 55 such pairs, so the total is 5×50=250.5 \times 50 = 250.

Thus, the correct answer is E .

2.

What is the value of

210+4100+61000?\frac{2}{10} + \frac{4}{100} + \frac{6}{1000}?

0.0120.012

0.02460.0246

0.120.12

0.2460.246

246246

Difficulty rating: 450
Small Hint:

Write each fraction as a decimal using its place value

Big Hint:

210=0.2,\frac{2}{10} = 0.2, 4100=0.04,\frac{4}{100} = 0.04, and 61000=0.006\frac{6}{1000} = 0.006

Solution:

Each fraction is a decimal in a different place: 210=0.2,\frac{2}{10} = 0.2, 4100=0.04,\frac{4}{100} = 0.04, and 61000=0.006.\frac{6}{1000} = 0.006.

Adding these gives 0.2+0.04+0.006=0.246.0.2 + 0.04 + 0.006 = 0.246.

Thus, the correct answer is D .

3.

Which of the following numbers is the largest?

0.990.99

0.90990.9099

0.90.9

0.9090.909

0.90090.9009

Difficulty rating: 450
Small Hint:

Give every number the same number of decimal places by appending zeros

Big Hint:

Compare digit by digit from the left; the hundredths place decides between the leading 0.90.9 numbers

Solution:

Write each number with four decimal places: 0.9900,0.9900, 0.9099,0.9099, 0.9000,0.9000, 0.9090,0.9090, 0.9009.0.9009.

Comparing from the left, 0.99000.9900 has the largest hundredths digit, so 0.990.99 is the largest.

Thus, the correct answer is A .

4.

Estimate to determine which of the following is closest to

4010.205.\frac{401}{0.205}.

0.20.2

22

2020

200200

20002000

Concepts:estimation
Difficulty rating: 660
Small Hint:

Round 401401 and 0.2050.205 to simple numbers

Big Hint:

4010.2054000.2\frac{401}{0.205} \approx \frac{400}{0.2}

Solution:

Round the numerator and denominator: 4010.2054000.2.\frac{401}{0.205} \approx \frac{400}{0.2}.

Since 4000.2=2000,\frac{400}{0.2} = 2000, the value is closest to 2000.2000.

Thus, the correct answer is E .

5.

What is the value of

15+9×(6÷3)?-15 + 9 \times (6 \div 3)?

48-48

12-12

3-3

33

1212

Difficulty rating: 560
Small Hint:

Follow the order of operations: parentheses first, then multiplication, then addition

Big Hint:

6÷3=2,6 \div 3 = 2, so the expression becomes 15+9×2-15 + 9 \times 2

Solution:

Inside the parentheses, 6÷3=2.6 \div 3 = 2. Then multiplication comes before addition, so 9×2=18.9 \times 2 = 18.

Finally, 15+18=3.-15 + 18 = 3.

Thus, the correct answer is D .

6.

If the markings on the number line shown are equally spaced, what is the number y?y?

33

1010

1212

1515

1616

Difficulty rating: 660
Small Hint:

Count how many equal spaces lie between 00 and 2020

Big Hint:

There are 55 spaces from 00 to 20,20, so each space is 205=4;\frac{20}{5} = 4; then count the spaces up to yy

Solution:

From 00 to 2020 there are 55 equal spaces, so each space is 205=4.\frac{20}{5} = 4.

The mark labeled yy is 33 spaces from 0,0, so y=3×4=12.y = 3 \times 4 = 12.

Thus, the correct answer is C .

7.

The value of 2020 quarters and 1010 dimes equals the value of 1010 quarters and nn dimes. What is n?n?

1010

2020

3030

3535

4545

Difficulty rating: 770
Small Hint:

First find the total value of 2020 quarters and 1010 dimes

Big Hint:

After using 1010 quarters, find how much value the remaining dimes must supply, then divide by 1010¢

Solution:

The value of 2020 quarters and 1010 dimes is $5.00+$1.00=$6.00.\$5.00 + \$1.00 = \$6.00.

Ten quarters are worth $2.50,\$2.50, so the nn dimes must supply $6.00$2.50=$3.50.\$6.00 - \$2.50 = \$3.50. That is 3535 dimes, so n=35.n = 35.

Thus, the correct answer is D .

8.

What is the value of

(2×3×4)(12+13+14)?(2 \times 3 \times 4)\left(\frac{1}{2} + \frac{1}{3} + \frac{1}{4}\right)?

11

33

99

2424

2626

Difficulty rating: 820
Small Hint:

2×3×4=24;2 \times 3 \times 4 = 24; distribute it across the three fractions

Big Hint:

2412+2413+241424 \cdot \frac{1}{2} + 24 \cdot \frac{1}{3} + 24 \cdot \frac{1}{4}

Solution:

Since 2×3×4=24,2 \times 3 \times 4 = 24, distribute it over the sum: 2412+2413+2414.24 \cdot \frac{1}{2} + 24 \cdot \frac{1}{3} + 24 \cdot \frac{1}{4}.

This equals 12+8+6=26.12 + 8 + 6 = 26.

Thus, the correct answer is E .

9.

There are 22 boys for every 33 girls in Ms. Johnson’s math class. If there are 3030 students in her class, what percent of them are boys?

12%12\%

20%20\%

40%40\%

60%60\%

6623%66\dfrac{2}{3}\%

Difficulty rating: 660
Small Hint:

For every 22 boys there are 33 girls, so boys are 22 out of every 55 students

Big Hint:

Convert the fraction 25\frac{2}{5} to a percent

Solution:

Boys make up 22 out of every 2+3=52 + 3 = 5 students, which is 25\frac{2}{5} of the class. The total of 3030 students is not even needed.

As a percent, 25=40%.\frac{2}{5} = 40\%.

Thus, the correct answer is C .

10.

How many degrees are in the smaller angle between the hour hand and the minute hand of a clock that reads seven o’clock?

5050^\circ

120120^\circ

135135^\circ

150150^\circ

165165^\circ

Concepts:clock
Difficulty rating: 730
Small Hint:

The 1212 hour marks split the clock face into 1212 equal angles

Big Hint:

Each hour mark is 30;30^\circ; at seven o’clock the hands are 55 marks apart the short way

Solution:

The 1212 numbers divide the clock into 1212 equal sections of 36012=30\frac{360^\circ}{12} = 30^\circ each.

At seven o’clock the hands point to 1212 and 7,7, which are 55 sections apart the short way, giving 5×30=150.5 \times 30^\circ = 150^\circ.

Thus, the correct answer is D .

11.

Which of the five “T-like shapes” would be symmetric to the one shown with respect to the dashed line?

Difficulty rating: 860
Small Hint:

Two figures are symmetric across the dashed line when folding the paper along that line makes one land exactly on the other

Big Hint:

A reflection across a vertical line keeps top and bottom fixed but swaps left and right; track where the small corner square ends up and how the slanted strokes turn

Solution:

Two figures are symmetric with respect to the dashed line when folding the paper along that line makes one coincide with the other. Reflecting across the vertical dashed line swaps left and right while leaving top and bottom unchanged.

Under this reflection the small square in the top-left corner moves to the top-right corner, the straight crossbar reverses its slant, and the stem that points down and to the right must instead point down and to the left. Only one choice has all three of these features.

Thus, the correct answer is B .

12.

What is the value of

113112?\frac{1 - \frac{1}{3}}{1 - \frac{1}{2}}?

13\dfrac{1}{3}

23\dfrac{2}{3}

34\dfrac{3}{4}

32\dfrac{3}{2}

43\dfrac{4}{3}

Concepts:fraction
Difficulty rating: 770
Small Hint:

Simplify the numerator and denominator separately

Big Hint:

113=231 - \frac{1}{3} = \frac{2}{3} and 112=12;1 - \frac{1}{2} = \frac{1}{2}; then divide

Solution:

The numerator is 113=23,1 - \frac{1}{3} = \frac{2}{3}, and the denominator is 112=12.1 - \frac{1}{2} = \frac{1}{2}.

Dividing gives 23÷12=23×2=43.\frac{2}{3} \div \frac{1}{2} = \frac{2}{3} \times 2 = \frac{4}{3}.

Thus, the correct answer is E .

13.

Which of the following is equal to

97×53?\frac{9}{7 \times 53}?

0.90.7×53\dfrac{0.9}{0.7 \times 53}

0.90.7×0.53\dfrac{0.9}{0.7 \times 0.53}

0.90.7×5.3\dfrac{0.9}{0.7 \times 5.3}

0.97×0.53\dfrac{0.9}{7 \times 0.53}

0.090.07×0.53\dfrac{0.09}{0.07 \times 0.53}

Difficulty rating: 920
Small Hint:

Dividing the numerator and one factor of the denominator by the same number keeps the fraction unchanged

Big Hint:

To turn 99 into 0.9,0.9, divide by 10;10; to keep the value equal, divide one factor of the denominator by 1010 as well

Solution:

To rewrite the numerator 99 as 0.9,0.9, divide it by 10.10. To keep the fraction equal, divide the denominator by 1010 too, which means dividing one of its factors by 10.10.

Dividing the factor 77 by 1010 gives 0.90.7×53,\frac{0.9}{0.7 \times 53}, which equals the original. Each other choice changes the value by a factor of 1010 or 100.100.

Thus, the correct answer is A .

14.

When placing each of the digits 2,2, 4,4, 5,5, 6,6, 99 in exactly one of the boxes of this subtraction problem, what is the smallest difference that is possible?

5858

123123

149149

171171

176176

Difficulty rating: 860
Small Hint:

Make the three-digit number as small as possible and the two-digit number as large as possible

Big Hint:

The largest two-digit number from these digits uses 99 and 6;6; build the smallest three-digit number from the digits that remain

Solution:

The difference is smallest when the three-digit number (the top) is as small as possible and the two-digit number (subtracted) is as large as possible.

The largest two-digit number is 96,96, using the digits 99 and 6.6. The smallest three-digit number from the remaining digits 2,2, 4,4, 55 is 245.245. So the smallest difference is 24596=149.245 - 96 = 149.

Thus, the correct answer is C .

15.

In parallelogram ABCDABCD shown, what is the area of the shaded region BEDC?BEDC?

2424

4848

6060

6464

8080

Difficulty rating: 860
Small Hint:

The shaded region is the whole parallelogram with the triangle ABEABE removed

Big Hint:

The parallelogram area is 10×8;10 \times 8; triangle ABEABE has base AE=106AE = 10 - 6 and height 88

Solution:

The parallelogram has base 1010 and height 8,8, so its area is 10×8=80.10 \times 8 = 80.

The unshaded triangle ABEABE has base AE=ADED=106=4AE = AD - ED = 10 - 6 = 4 and height 8,8, so its area is 12×4×8=16.\frac{1}{2} \times 4 \times 8 = 16. The shaded area is 8016=64.80 - 16 = 64.

Thus, the correct answer is D .

16.

In how many ways can 4747 be written as the sum of two primes?

00

11

22

33

more than 33

Concepts:primeparity
Difficulty rating: 860
Small Hint:

4747 is odd, so one of the two primes must be even

Big Hint:

The only even prime is 2,2, which would force the other prime to be 4545

Solution:

Since 4747 is odd, a sum of two primes equal to 4747 needs one even prime and one odd prime. The only even prime is 2.2.

That would require the other number to be 472=45,47 - 2 = 45, but 45=9×545 = 9 \times 5 is not prime. So there is no way.

Thus, the correct answer is A .

17.

The number NN is between 99 and 17.17. Which of the following could be the average of 6,6, 10,10, and N?N?

88

1010

1212

1414

1616

Difficulty rating: 800
Small Hint:

The average is 6+10+N3;\frac{6 + 10 + N}{3}; find its value at each end of the range for NN

Big Hint:

As NN runs from 99 to 17,17, the average runs from 253\frac{25}{3} to 1111

Solution:

The average is 6+10+N3=16+N3.\frac{6 + 10 + N}{3} = \frac{16 + N}{3}. When N=9N = 9 this is 2538.3,\frac{25}{3} \approx 8.3, and when N=17N = 17 it is 11.11.

So the average lies strictly between about 8.38.3 and 11.11. Among the choices, only 1010 falls in this range.

Thus, the correct answer is B .

18.

A calculator has a reciprocal key that replaces the number currently displayed with its reciprocal. For example, if the display shows 44 and the key is pressed, the display becomes 0.25.0.25. If 3232 is currently displayed, what is the fewest number of times the reciprocal key must be pressed so that the display again reads 32?32?

11

22

33

44

55

Difficulty rating: 730
Small Hint:

Pressing the key once turns 3232 into 132\frac{1}{32}

Big Hint:

The reciprocal of the reciprocal of a number is the number itself

Solution:

Pressing the key once changes 3232 to its reciprocal 132.\frac{1}{32}.

Pressing it a second time takes the reciprocal again, returning to 1132=32.\frac{1}{\frac{1}{32}} = 32. So 22 presses are enough.

Thus, the correct answer is B .

19.

The graph below shows the total accumulated dollars (in millions) spent by the Surf City government during 1988.1988. For example, about 0.50.5 million had been spent by the beginning of February and approximately 22 million by the end of April. Approximately how many millions of dollars were spent during the summer months of June, July, and August?

1.51.5

2.52.5

3.53.5

4.54.5

5.55.5

Difficulty rating: 920
Small Hint:

Because the graph shows an accumulated total, the amount spent over a span of months is the increase in that total across the span

Big Hint:

Read the height of the curve at the beginning of June and at the end of August, then subtract the two values

Solution:

The graph gives the total accumulated spending, so the amount spent during June, July, and August equals the accumulated total at the end of August minus the accumulated total at the beginning of June.

The curve is at a bit more than 22 million at the beginning of June and a bit more than 4.54.5 million by the end of August. The difference is about 4.52=2.54.5 - 2 = 2.5 million.

Thus, the correct answer is B .

20.

The figure shown may be folded along the lines to form a number cube. Three faces come together at each corner of the cube. What is the largest sum of three numbers whose faces come together at a corner?

1111

1212

1313

1414

1515

Difficulty rating: 920
Small Hint:

A corner uses three faces that are mutually adjacent, so no two of them can be opposite faces

Big Hint:

Folding the net, the opposite face pairs are 11 & 3,3, 22 & 5,5, and 44 & 6;6; take the larger of each pair

Solution:

When the net is folded, the pairs of opposite faces are 11 and 3,3, 22 and 5,5, and 44 and 6.6. Three faces meeting at a corner must come one from each opposite pair.

To maximize the sum, take the larger number from each pair: 3,3, 5,5, and 6.6. These three faces do meet at a corner, and their sum is 3+5+6=14.3 + 5 + 6 = 14.

Thus, the correct answer is D .

21.

Jack had a bag of 128128 apples. He sold 25%25\% of them to Jill. Next he sold 25%25\% of those remaining to June. Of those apples still in his bag, he gave the shiniest one to his teacher. How many apples did Jack have then?

77

6363

6565

7171

111111

Concepts:percentage
Difficulty rating: 860
Small Hint:

Selling 25%25\% leaves 75%=3475\% = \frac{3}{4} of the apples

Big Hint:

Multiply 128128 by 34\frac{3}{4} twice, then subtract the one given to the teacher

Solution:

After selling 25%25\% to Jill, Jack keeps 34×128=96\frac{3}{4} \times 128 = 96 apples. After selling 25%25\% of those to June, he keeps 34×96=72\frac{3}{4} \times 96 = 72 apples.

He then gives 11 to his teacher, leaving 721=71.72 - 1 = 71.

Thus, the correct answer is D .

22.

The letters A,A, J,J, H,H, S,S, M,M, EE and the digits 1,1, 9,9, 8,8, 99 are each cycled separately (shifted one place at a time) to build a numbered list. Starting from AJHSME 1989,1989, the list begins: line 11 is JHSMEA 9891,9891, line 22 is HSMEAJ 8919,8919, line 33 is SMEAJH 9198,9198, and so on. On what numbered line will AJHSME 19891989 appear for the first time?

66

1010

1212

1818

2424

Difficulty rating: 1020
Small Hint:

The six letters return to AJHSME every 66 lines; the four digits return to 19891989 every 44 lines

Big Hint:

Both return together on the least common multiple of 66 and 44

Solution:

The six letters cycle back to their original order AJHSME every 66 lines, and the four digits cycle back to 19891989 every 44 lines.

Both happen on the same line at the least common multiple of 66 and 4,4, which is 12.12. So AJHSME 19891989 first reappears on line 12.12.

Thus, the correct answer is C .

23.

An artist has 1414 cubes, each with an edge of 11 meter. She stands them on the ground to form a sculpture as shown. She then paints the exposed surface of the sculpture. How many square meters does she paint?

2121

2424

3333

3737

4242

Difficulty rating: 1050
Small Hint:

Count the exposed faces on the four vertical sides, then the faces seen from directly above

Big Hint:

Each of the four sides shows 66 exposed faces, and looking straight down covers the full 3×33 \times 3 footprint of top faces

Solution:

Because each higher layer is flush into one corner, each of the four vertical sides shows a stepped profile of 3+2+1=63 + 2 + 1 = 6 exposed square faces, for 4×6=244 \times 6 = 24 side faces.

Viewed from directly above, the top faces cover the full 3×33 \times 3 footprint, adding 99 more faces. The bottom rests on the ground and is not painted, so the total is 24+9=3324 + 9 = 33 square meters.

Thus, the correct answer is C .

24.

Suppose a square piece of paper is folded in half vertically. The folded paper is then cut in half along the dashed line. Three rectangles are formed—a large one and two small ones. What is the ratio of the perimeter of one of the small rectangles to the perimeter of the large rectangle?

12\dfrac{1}{2}

23\dfrac{2}{3}

34\dfrac{3}{4}

45\dfrac{4}{5}

56\dfrac{5}{6}

Difficulty rating: 950
Small Hint:

Give the original square a convenient side length, such as 4,4, and track the pieces

Big Hint:

The piece containing the fold unfolds to a 2×42 \times 4 large rectangle, while each other layer is a 1×41 \times 4 small rectangle

Solution:

Let the square have side 4.4. Folding in half makes a 2×42 \times 4 stack of two layers. Cutting parallel to the fold splits it into a strip containing the fold and a strip that does not.

The strip with the fold unfolds into the large rectangle, 2×4,2 \times 4, with perimeter 2(2+4)=12.2(2 + 4) = 12. The other strip is two separate small rectangles, each 1×4,1 \times 4, with perimeter 2(1+4)=10.2(1 + 4) = 10. The ratio is 1012=56.\frac{10}{12} = \frac{5}{6}.

Thus, the correct answer is E .

25.

Every time the two wheels shown are spun, two numbers are selected by the pointers. What is the probability that the sum of the two selected numbers is even?

16\dfrac{1}{6}

37\dfrac{3}{7}

12\dfrac{1}{2}

23\dfrac{2}{3}

57\dfrac{5}{7}

Difficulty rating: 920
Small Hint:

A sum is even exactly when both numbers are even or both are odd

Big Hint:

The first wheel has 22 even and 22 odd numbers; the second has 11 even and 22 odd

Solution:

The sum is even when both numbers are even or both are odd. The first wheel has evens {4,8}\{4, 8\} and odds {3,5},\{3, 5\}, each with probability 24=12.\frac{2}{4} = \frac{1}{2}. The second wheel has even {6}\{6\} with probability 13\frac{1}{3} and odds {7,9}\{7, 9\} with probability 23.\frac{2}{3}.

So the probability of an even sum is 1213+1223=16+26=12.\frac{1}{2} \cdot \frac{1}{3} + \frac{1}{2} \cdot \frac{2}{3} = \frac{1}{6} + \frac{2}{6} = \frac{1}{2}.

Thus, the correct answer is C .