1999 AMC 8 Problems

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Timed

40:00

1.

(6 ? 3)+4(21)=5.(6 \ ? \ 3) + 4 - (2 - 1) = 5. To make this statement true, the question mark between the 66 and the 33 should be replaced by

÷\div

×\times

++

-

None of these

Answer: A
Concepts:order of operations
Difficulty rating: 370
Small Hint:

Simplify the part without the question mark first

Big Hint:

Find the operation that makes 6 ? 3=26\ ?\ 3=2

Solution:

We have that 4(21)=41=3. 4 - (2 - 1) = 4 - 1 = 3.

Then (6 ? 3)+3=5 (6 \ ? \ 3) + 3 = 5 6 ? 3=2. 6 \ ? \ 3 = 2.

The only choice that works is ÷.\div.

Thus, A is the correct answer.

2.

What is the degree measure of the smaller angle formed by the hands of a clock at 1010 o’clock?

3030

4545

6060

7575

9090

Answer: C
Concepts:clock
Difficulty rating: 450
Small Hint:

Each hour mark is 3030^\circ apart

Big Hint:

At 1010 o’clock, the hands are two hour marks apart

Solution:

At 1010 o’clock, we have that the hour hand is at 1010 and the minute hand is at 12.12.

This means that the hands are a 16\frac{1}{6} of the entire circle apart. This is equal to 36016=60. 360^{\circ} \cdot \dfrac{1}{6} = 60^{\circ}.

Thus, C is the correct answer.

3.

Which triplet of numbers has a sum NOT equal to 1?1?

(12,13,16)(\frac12,\frac13,\frac16)

(2,2,1)(2,-2,1)

(0.1,0.3,0.6)(0.1,0.3,0.6)

(1.1,2.1,1.0)(1.1,-2.1,1.0)

(32,52,5)(-\frac32,-\frac52,5)

Answer: D
Difficulty rating: 560
Small Hint:

Compute the easiest-looking sums first

Big Hint:

Only one listed triplet fails to total 11

Solution:

We have that A is the same as 36+26+16=1. \dfrac{3}{6} + \dfrac{2}{6} + \dfrac{1}{6} = 1.

B reduces to 22+1=1. 2 - 2 + 1 = 1.

C adds up to 0.1+0.3+0.6=1. 0.1 + 0.3 + 0.6 = 1.

D however adds to 1.12.1+1.0=0. 1.1 - 2.1 + 1.0 = 0.

To make sure, we check that E has a sum of 82+5=54=1. -\dfrac{8}{2} + 5 = 5 - 4 = 1.

Thus, D is the correct answer.

4.

The diagram shows the miles traveled by bikers Alberto and Bjorn. After four hours, about how many more miles has Alberto biked than Bjorn?

1515

2020

2525

3030

3535

Answer: A
Difficulty rating: 660
Small Hint:

Read both distances at 44 hours

Big Hint:

Subtract Bjorn’s distance from Alberto’s

Solution:

Looking at the graph, we have that Alberto has biked 6060 miles and Bjorn 4545 miles after 44 hours.

The difference between the two distances is 6045=15 60 - 45 = 15 miles.

Thus, A is the correct answer.

5.

A rectangular garden 5050 feet long and 1010 feet wide is enclosed by a fence. To make the garden larger, while using the same fence, its shape is changed to a square. By how many square feet does this enlarge the garden?

100100

200200

300300

400400

500500

Answer: D
Concepts:perimeterarea
Difficulty rating: 770
Small Hint:

Keep the perimeter the same

Big Hint:

The square side length is one-fourth of the old perimeter

Solution:

The current length of the fence is 2(50+10)=260=120 2(50 + 10) = 2 \cdot 60 = 120 feet. If all the sides become the same, then each side length is 120÷4=30 120 \div 4 = 30 feet. As such, the area of the square is then 302=900 30^2 = 900 square feet. Note that the original area of the garden is 5010=500 50 \cdot 10 = 500 square feet.

Therefore, the difference is 900500=400 900 - 500 = 400 square feet.

Thus, D is the correct answer.

6.

Bo, Coe, Flo, Jo, and Moe have different amounts of money. Neither Jo nor Bo has as much money as Flo. Both Bo and Coe have more than Moe. Jo has more than Moe, but less than Bo. Who has the least amount of money?

Bo

Coe

Flo

Jo

Moe

Answer: E
Difficulty rating: 820
Small Hint:

Eliminate anyone known to have more than someone else

Big Hint:

Use each comparison to show who is above Moe

Solution:

Let B,C,F,J,B, C, F, J, and MM represent the amount of money that Bo, Coe, Flo, Jo, and Moe have respectively.

Then J<F,B<F. J \lt F,\quad B \lt F.

We also have B>M,C>M. B \gt M,\quad C \gt M.

Finally, we are given M<J<B. M \lt J \lt B.

From the first and second inequalities, we can get that F>M.F \gt M.

This shows that everyone has more money than Moe.

Thus, E is the correct answer.

7.

The third exit on a highway is located at milepost 4040 and the tenth exit is at milepost 160.160. There is a service center on the highway located three-fourths of the way from the third exit to the tenth exit. At what milepost would you expect to find this service center?

9090

100100

110110

120120

130130

Answer: E
Concepts:fraction
Difficulty rating: 900
Small Hint:

First find the distance between mileposts 4040 and 160160

Big Hint:

Move 34\frac{3}{4} of that distance past milepost 4040

Solution:

There are 16040=120160 - 40 = 120 miles between the third and tenth exits.

This means 34\frac{3}{4} of the way is at the 40+34120=130 40 + \dfrac{3}{4} \cdot 120 = 130 milepost.

Thus, E is the correct answer.

8.

Six squares are colored, front and back (RR = red, BB = blue, OO = orange, YY = yellow, GG = green, and WW = white). They are hinged together as shown, then folded to form a cube. The face opposite the white face is

BB

GG

OO

RR

YY

Answer: A
Difficulty rating: 960
Small Hint:

Fold around the square marked YY

Big Hint:

Track which face lands opposite WW

Solution:

Consider the cube with the yellow face facing upwards.

Then we can fold the white, green, and orange faces down.

Since the blue face is attached to the green face, it will end up folding backwards.

This means that the blue face will end up facing opposite the white face.

Thus, A is the correct answer.

9.

Three flower beds overlap as shown. Bed AA has 500500 plants, bed BB has 450450 plants, and bed CC has 350350 plants. Beds AA and BB share 5050 plants, while beds AA and CC share 100.100. The total number of plants is

850850

10001000

11501150

13001300

14501450

Answer: C
Difficulty rating: 1020
Small Hint:

Add the three bed counts first

Big Hint:

Subtract the overlaps that were counted twice

Solution:

Note there are 50+100=15050 + 100 = 150 plants that are in two beds and there are no plants in all three beds.

The total number of plants is then 500+450+350150=1150. 500 + 450 + 350 - 150 = 1150. We subtract to get rid of the plants that we counted twice.

Thus, C is the correct answer.

10.

A complete cycle of a traffic light takes 6060 seconds. During each cycle the light is green for 2525 seconds, yellow for 55 seconds, and red for 3030 seconds. At a randomly chosen time, what is the probability that the light will NOT be green?

14\dfrac{1}{4}

13\dfrac{1}{3}

512\dfrac{5}{12}

12\dfrac{1}{2}

712\dfrac{7}{12}

Answer: E
Difficulty rating: 980
Small Hint:

Not green means red or yellow

Big Hint:

Use time not green over total cycle time

Solution:

During a given cycle, the light is not green for 30+5=3530 + 5 = 35 seconds.

Then the probability that it is not green is 3560=712. \dfrac{35}{60} = \dfrac{7}{12}.

Thus, E is the correct answer.

11.

Each of the five numbers 1,1, 4,4, 7,7, 10,10, and 1313 is placed in one of the five squares so that the sum of the three numbers in the horizontal row equals the sum of the three numbers in the vertical column. The largest possible value for the horizontal or vertical sum is

2020

2121

2222

2424

3030

Answer: D
Difficulty rating: 1140
Small Hint:

The center square is counted in both sums

Big Hint:

Maximize the center number, then split the doubled total equally

Solution:

Let xx be the number in the center. The horizontal sum plus the vertical sum counts every number once, except the center number is counted twice.

So twice the common sum is 1+4+7+10+13+x1+4+7+10+13+x =35+x=35+x. This is largest when x=13x=13.

The largest common sum is therefore at most 35+132=24\dfrac{35+13}{2}=24, and it is attainable: put 1313 in the center, 11 and 1010 at the ends of one row, and 44 and 77 at the ends of the other. Both sums are 2424.

Thus, D is the correct answer.

12.

The ratio of the number of games won to the number of games lost (no ties) by the Middle School Middies is 114.\frac{11}{4}. To the nearest whole percent, what percent of its games did the team lose?

24%24\%

27%27\%

36%36\%

45%45\%

73%73\%

Answer: B
Difficulty rating: 1120
Small Hint:

Use 1111 wins and 44 losses as one batch

Big Hint:

The lost fraction is losses over total games

Solution:

The team can be viewed as winning 1111 games and losing 44 games in each batch of 1515 games.

The percent lost is 415100%27%\dfrac4{15}\cdot100\%\approx 27\%.

Thus, B is the correct answer.

13.

The average age of the 4040 members of a computer science camp is 1717 years. There are 2020 girls, 1515 boys, and 55 adults. If the average age of the girls is 1515 and the average age of the boys is 16,16, what is the average age of the adults?

2626

2727

2828

2929

3030

Answer: C
Concepts:mean
Difficulty rating: 1150
Small Hint:

Convert each average into a total age

Big Hint:

Subtract the girls’ and boys’ totals from the camp total

Solution:

The sum of the ages of everybody at the camp is 4017=680. 40 \cdot 17 = 680.

The sum of the ages of the girls is 2015=300 20 \cdot 15 = 300 and of the boys is 1516=240. 15 \cdot 16 = 240.

As such, we know that the ages of the adults must be 680300240=140. 680 - 300 - 240 = 140.

And therefore, the average age of the adults is then 140÷5=28. 140 \div 5 = 28.

Thus, C is the correct answer.

14.

In trapezoid ABCD,ABCD, the sides ABAB and CDCD are equal. The perimeter of ABCDABCD is

2727

3030

3232

3434

4848

Answer: D
Difficulty rating: 1220
Small Hint:

Drop an altitude from BB

Big Hint:

The two side right triangles have horizontal leg 44

Solution:

Let HH be where the altitude from BB to AD\overline{AD} intersects AD.\overline{AD}.

Then we have that AH=1682=4, AH = \dfrac{16 - 8}{2} = 4, since AB=CD.AB = CD.

We then have that AB=42+32=25=5. AB = \sqrt{4^2 + 3^2} = \sqrt{25} = 5.

Then the perimeter is 8+16+25=34. 8 + 16 + 2 \cdot 5 = 34.

Thus, D is the correct answer.

15.

Bicycle license plates in Flatville each contain three letters. The first is chosen from the set {C,H,L,P,R},\{C,H,L,P,R\}, the second from {A,I,O},\{A,I,O\}, and the third from {D,M,N,T}.\{D,M,N,T\}.

When Flatville needed more license plates, they added two new letters. The new letters may both be added to one set or one letter may be added to one set and one to another set. What is the largest possible number of additional license plates that can be made by adding two letters?

2424

3030

3636

4040

6060

Answer: D
Difficulty rating: 1330
Small Hint:

The old number of plates is 5345\cdot3\cdot4

Big Hint:

To maximize a product, make the three factors as balanced as possible

Solution:

There are currently 534=605 \cdot 3 \cdot 4 = 60 license plates that can be made.

If both letters are added to the first set, then there are 734=84 7 \cdot 3 \cdot 4 = 84 possible plates.

If they are both added to the second, there are 554=100 5 \cdot 5 \cdot 4 = 100 plates.

If they are added to the third, there are 536=90 5 \cdot 3 \cdot 6 = 90 choices.

If one is added to the first set and the other to the second set, there are 644=96 6 \cdot 4 \cdot 4 = 96 plates.

If the other is added to the third set, we get 635=90 6 \cdot 3 \cdot 5 = 90 possible plates.

Finally, if the letters are added to the second and third sets, there are 545=100 5 \cdot 4 \cdot 5 = 100 plates.

We see that 100100 is the greatest number of plates that we can achieve. This is an additional 10060=40100 - 60 = 40 plates.

Thus, D is the correct answer.

16.

Tori’s mathematics test had 7575 problems: 1010 arithmetic, 3030 algebra, and 3535 geometry problems. Although she answered 70%70\% of the arithmetic, 40%40\% of the algebra, and 60%60\% of the geometry problems correctly, she did not pass the test because she got less than 60%60\% of the problems right.

How many more problems would she have needed to answer correctly to earn a 60%60\% passing grade?

11

55

77

99

1111

Answer: B
Concepts:percentage
Difficulty rating: 1230
Small Hint:

Compute Tori’s current correct answers by category

Big Hint:

Compare that total with 60%60\% of 7575

Solution:

She answered 100.7=7 10 \cdot 0.7 = 7 arithmetic questions correctly. She answered 300.4=12 30 \cdot 0.4 = 12 algebra ones correctly. She also got 350.6=2135 \cdot 0.6 = 21 geometry questions correctly.

This means she got a total of 7+12+21=40 7 + 12 + 21 = 40 questions correct.

As such, in order to get a 60%,60\%, Tori must have answered 750.6=45 75 \cdot 0.6 = 45 questions correctly. This means that she would have needed to answer an additional 4540=545 - 40 = 5 questions correctly.

Thus, B is the correct answer.

17.

Problems 17,17, 18,18, and 1919 refer to the following:

Cookies For a Crowd

At Central Middle School the 108108 students who take the AMC 88 meet in the evening to talk about problems and eat an average of two cookies apiece. Walter and Gretel are baking Bonnie’s Best Bar Cookies this year. Their recipe, which makes a pan of 1515 cookies, lists these items: 1121\dfrac{1}{2} cups of flour, 22 eggs, 33 tablespoons butter, 34\dfrac{3}{4} cups sugar, and 11 package of chocolate drops. They will make only full recipes, not partial recipes.

Walter can buy eggs by the half-dozen. How many half-dozens should he buy to make enough cookies? (Some eggs and some cookies may be left over.)

11

22

55

77

1515

Answer: C
Difficulty rating: 1250
Small Hint:

Find how many full recipes are needed for 216216 cookies

Big Hint:

Each full recipe uses 22 eggs

Solution:

Since the students eat an average of 22 cookies each, they will eat a total of 1082=216108 \cdot 2 = 216 cookies.

Each recipe makes 1515 cookies, which means we need 21615=15 \left\lceil\dfrac{216}{15}\right\rceil = 15 full recipes to make enough cookies.

Each pan requires 22 eggs, which means we need 152=3015 \cdot 2 = 30 eggs. There are 66 eggs in a half-dozen, so we need 30÷6=530 \div 6 = 5 half-dozens.

Thus, C is the correct answer.

18.

They learn that a big concert is scheduled for the same night and attendance will be down 25%.25\%. How many recipes of cookies should they make for their smaller party?

66

88

99

1010

1111

Answer: E
Difficulty rating: 1310
Small Hint:

A 25%25\% drop leaves 75%75\% attendance

Big Hint:

Round the needed number of recipes up

Solution:

There will now only be 108(114)=10834=81 108 \cdot \left(1 - \dfrac{1}{4}\right) = 108 \cdot \dfrac{3}{4} = 81 people at the party. This means we need 81215=16215=11 \left\lceil \dfrac{81 \cdot 2}{15} \right\rceil = \left\lceil \dfrac{162}{15} \right\rceil = 11 recipes.

Thus, E is the correct answer.

19.

The drummer gets sick. The concert is cancelled. Walter and Gretel must make enough pans of cookies to supply 216216 cookies. There are 88 tablespoons in a stick of butter. How many sticks of butter will be needed? (Some butter may be left over, of course.)

55

66

77

88

99

Answer: B
Difficulty rating: 1340
Small Hint:

First round the number of pans up

Big Hint:

Convert tablespoons of butter to sticks and round up

Solution:

To make 216216 cookies, they have to make 21615=15 \left\lceil \dfrac{216}{15} \right\rceil = 15 pans. Since each pan requires 33 tablespoons of butter, all the pans will need 153=4515 \cdot 3 = 45 tablespoons.

They will then need 458=6 \left\lceil \dfrac{45}{8} \right\rceil = 6 sticks of butter.

Thus, B is the correct answer.

20.

Figure 11 is called a “stack map.” The numbers tell how many cubes are stacked in each position. Fig. 22 shows these cubes, and Fig. 33 shows the view of the stacked cubes as seen from the front.

Which of the following is the front view for the stack map in Fig. 4?4?

Answer: B
Concepts:3D geometry
Difficulty rating: 1410
Small Hint:

A front view keeps only the taller stack in each column

Big Hint:

For each column, take the larger front/back height

Solution:

Note that the height of the column is the maximum height of the front and back columns.

In the left column, we have that the back column is taller with height 2.2.

In the middle column, we have that the front column is taller with height 3.3.

Finally, the right column has height 4.4.

Thus, B is the correct answer.

21.

The degree measure of angle AA is

2020

3030

3535

4040

4545

Answer: B
Concepts:angle chasing
Difficulty rating: 1520
Small Hint:

Use supplementary angles at the two intersections

Big Hint:

Then use a triangle angle sum near the 4040^\circ angle

Solution:

Label the vertices as below.

We then have that ABC=180100=80 \angle ABC = 180^{\circ} - 100^{\circ} = 80^{\circ} by supplementary angles. Then we have CED=180110=70 \angle CED = 180^{\circ} - 110^{\circ} = 70^{\circ} again by supplementary angles. Using the sum of the interior angles of a triangle is 180,180^{\circ}, we get ECD=1807040=70. \begin{aligned} \angle ECD &= 180^{\circ} - 70^{\circ} - 40^{\circ} \\ &= 70^{\circ}. \end{aligned} Then, using vertical angles, we have ACB=ECD=70. \angle ACB = \angle ECD = 70^{\circ}. Finally, using the sum of the interior angles of a triangle, we get A=1808070=30. \angle A = 180^{\circ} - 80^{\circ} - 70^{\circ} = 30^{\circ}.

Thus, B is the correct answer.

22.

In a far-off land three fish can be traded for two loaves of bread and a loaf of bread can be traded for four bags of rice. How many bags of rice is one fish worth?

38\dfrac{3}{8}

12\dfrac{1}{2}

34\dfrac{3}{4}

2232\dfrac{2}{3}

3133\dfrac{1}{3}

Answer: D
Difficulty rating: 1180
Small Hint:

Turn each trade into a value equation

Big Hint:

Convert fish to bread, then bread to rice

Solution:

Three fish are worth two loaves of bread, so one fish is worth 23\dfrac23 of a loaf.

One loaf is worth 44 bags of rice, so one fish is worth 234=83=223\dfrac23\cdot4=\dfrac83=2\dfrac23 bags of rice.

Thus, D is the correct answer.

23.

Square ABCDABCD has sides of length 3.3. Segments CMCM and CNCN divide the square’s area into three equal parts. How long is segment CM?CM?

10\sqrt{10}

12\sqrt{12}

13\sqrt{13}

14\sqrt{14}

15\sqrt{15}

Answer: C
Difficulty rating: 1520
Small Hint:

Each of the three regions has area one-third of the square

Big Hint:

Use the area of BMC\triangle BMC to find BMBM

Solution:

The area of the square is 32=9,3^2 = 9, which means that the area of one region is 9÷3=3.9 \div 3 = 3.

This means the area of BMC\triangle BMC is 3,3, which means that 123BM=3 \dfrac{1}{2} \cdot 3 \cdot BM = 3 BM=2. BM = 2.

Since BMC\triangle BMC is right, we have CM=22+32=13. CM = \sqrt{2^2 + 3^2} = \sqrt{13}.

Thus, C is the correct answer.

24.

When 199920001999^{2000} is divided by 5,5, the remainder is

44

33

22

11

00

Answer: D
Difficulty rating: 1470
Small Hint:

Reduce 19991999 modulo 55

Big Hint:

Even powers of 1-1 modulo 55 have the same remainder

Solution:

Note that to find the remainder when divided by 5,5, we only care about the units digit.

This means we only have to observe how the powers of the units digit work, namely the powers of 9.9.

Then, looking at powers of 9,9, we see that the units digit alternates between 99 and 1:1: 9,81,729,. 9, 81, 729, \cdots. This means that 199920001999^{2000} ends in a 11 since the power is even.

The remainder when divided by 55 is then 1,1, since it is 11 more than a multiple of 10.10.

Thus, D is the correct answer.

25.

Points B,B, D,D, and JJ are midpoints of the sides of right triangle ACG.ACG. Points K,K, E,E, II are midpoints of the sides of triangle JDG,JDG, etc. If the dividing and shading process is done 100100 times (the first three are shown) and AC=CG=6,AC = CG = 6, then the total area of the shaded triangles is nearest

66

77

88

99

1010

Answer: A
Difficulty rating: 1620
Small Hint:

The shaded triangles form a geometric area pattern

Big Hint:

Each new shaded triangle has one-fourth the previous shaded area

Solution:

The first shaded triangle has legs 33 and 33, so its area is 1233=92\dfrac12\cdot3\cdot3=\dfrac92.

At each later step, the relevant triangle has half the leg length, so the shaded area is multiplied by (12)2=14\left(\dfrac12\right)^2=\dfrac14.

The total shaded area after many steps is therefore very close to the geometric sum 92(1+14+116+)=921114=6. \begin{gathered} \frac92\left(1+\frac14+\frac1{16}+\cdots\right) \\ {}= \frac92\cdot\frac1{1-\frac{1}{4}} \\ {}= 6. \end{gathered}

After 100100 steps the omitted tail is tiny, so the total area is nearest 66.

Thus, A is the correct answer.