2007 AMC 8 考试题目

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1.

Theresa's parents have agreed to buy her tickets to see her favorite band if she spends an average of 1010 hours per week helping around the house for 66 weeks. For the first 55 weeks she helps around the house for 8,8, 11,11, 7,7, 1212 and 1010 hours. How many hours must she work for the final week to earn the tickets?

99

1010

1111

1212

1313

Answer: D
Solution:

During the first 55 weeks, Theresa works for a total of 8+11+7+12+10=488+11+7+12+10=48 hours.

She promised to work 106=6010\cdot 6=60 hours in all.

This means that she has to work 6048=1260-48=12 hours during the final week to earn the tickets.

Thus, D is the correct answer.

2.

650650 students were surveyed about their pasta preferences. The choices were lasagna, manicotti, ravioli and spaghetti. The results of the survey are displayed in the bar graph. What is the ratio of the number of students who preferred spaghetti to the number of students who preferred manicotti?

25\dfrac{2}{5}

12\dfrac{1}{2}

54\dfrac{5}{4}

53\dfrac{5}{3}

52\dfrac{5}{2}

Answer: E
Solution:

There are 250250 students who preferred spaghetti and 100100 that preferred manicotti.

The ratio is therefore 250100=52. \dfrac{250}{100} = \dfrac{5}{2}.

Thus, E is the correct answer.

3.

What is the sum of the two smallest prime factors of 250?250?

22

55

77

1010

1212

Answer: C
Solution:

We can prime factorize 250250 to get 250=253. 250 = 2 \cdot 5^3. From this, we can see that 250250 only has two prime factors: 22 and 5.5. The sum of these is 7.7.

Thus, C is the correct answer.

4.

A haunted house has six windows. In how many ways can Georgie the Ghost enter the house by one window and leave by a different window?

1212

1515

1818

3030

3636

Answer: D
Solution:

Georgie has 66 options for which window he enters through. He, however, only has 55 options for the exit since it must be different from the entrance.

The total number of paths is therefore 65=30.6 \cdot 5 = 30.

Thus, D is the correct answer.

5.

Chandler wants to buy a $500\$500 mountain bike. For his birthday, his grandparents send him $50\$50, his aunt sends him $35\$35 and his cousin gives him $15\$15. He earns $16\$16 per week for his paper route. He will use all of his birthday money and all of the money he earns from his paper route. In how many weeks will he be able to buy the mountain bike?

2424

2525

2626

2727

2828

Answer: B
Solution:

The total amount of birthday money Chandler gets is 50+35+15=10050+35+15=100 dollars. Therefore, he still needs 500100=400500-100=400 dollars.

This can be earned in 400÷16=25400\div 16=25 weeks through his paper route.

Thus, B is the correct answer.

6.

The average cost of a long-distance call in the USA in 19851985 was 4141 cents per minute, and the average cost of a long-distance call in the USA in 20052005 was 77 cents per minute. Find the approximate percent decrease in the cost per minute of a long-distance call.

77

1717

3434

4141

8080

Answer: E
Solution:

The decrease in cost is 417=3441-7=34 cents per minute.

The percent decrease is 1003441%100\cdot\dfrac{34}{41}\%, which is a little more than 80%80\%, so the closest answer is 8080.

Thus, E is the correct answer.

7.

The average age of 55 people in a room is 3030 years. An 1818-year-old person leaves the room. What is the average age of the four remaining people?

2525

2626

2929

3333

3636

Answer: D
Solution:

Initially, the total age of everyone in the room is 530=1505 \cdot 30 = 150 years.

After the person leaves, the total age is 15018=132.150 - 18 = 132. With 44 people remaining, the average age becomes 1324=33.\dfrac{132}{4} = 33.

Thus, D is the correct answer.

8.

In trapezoid ABCD,ABCD, AD\overline{AD} is perpendicular to DC,\overline{DC}, AD=AB=3,AD = AB = 3, and DC=6.DC = 6. In addition, EE is on DC,\overline{DC}, and BE\overline{BE} is parallel to AD.\overline{AD}. Find the area of BEC.\triangle BEC.

33

4.54.5

66

99

1818

Answer: B
Solution:

We know that EC=DCDE EC = DC - DE=63 = 6 - 3 =3.= 3. We also know that BE=AD=3. BE = AD = 3. Therefore, the area of BEC\triangle BEC is 1233=92. \dfrac{1}{2} \cdot 3 \cdot 3 = \dfrac{9}{2}.

Thus, B is the correct answer.

9.

To complete the grid below, each of the digits 11 through 44 must occur once in each row and once in each column. What number will occupy the lower right-hand square?

11

22

33

44

cannot be determined\text{cannot be determined}

Answer: B
Solution:

The last square in the second row must be 11, since that row already contains 22 and 33, and the last column already contains 44.

Then the top-right square must be 33, since the top row already contains 11 and 22, and the last column already contains 11 and 44.

This forces the 22 to be in the bottom-right square.

Thus, B is the correct answer.

10.

For any positive integer n,n, define n\boxed{n} to be the sum of the positive factors of n.n. For example, 6=1+2+3+6=12. \boxed{6} = 1 + 2 + 3 + 6 = 12. Find 11.\boxed{\boxed{11}}.

1313

2020

2424

2828

3030

Answer: D
Solution:

First, we find that 11=1+11=12 \boxed{11} = 1 + 11 = 12 since 1111 is prime. Then we need to find 12.\boxed{12}. The factors of 1212 are 1,2,3,4,6,12. 1, 2, 3, 4, 6, 12. Adding these yields 12=28.\boxed{12} = 28.

Thus, D is the correct answer.

11.

Tiles I,II,IIII, II, III and IVIV are translated so one tile coincides with each of the rectangles A,B,CA, B, C and D.D. In the final arrangement, the two numbers on any side common to two adjacent tiles must be the same. Which of the tiles is translated to Rectangle C?C?

II

IIII

IIIIII

IVIV

cannot be determined\text{cannot be determined}

Answer: D
Solution:

Only Tile IIIIII has a 00 on an edge, so Tile IIIIII must be placed in CC or DD, where that edge can be on the outside.

Tile IIIIII also has a 55 on its right edge, and no other tile has a 55. Therefore Tile IIIIII must be placed in DD, with its 55 on the outside.

The only tile that can match the 11 on the left edge of Tile IIIIII is Tile IVIV, so Tile IVIV is placed in CC.

Thus, D is the correct answer.

12.

A unit hexagram is composed of a regular hexagon of side length 11 and its 66 equilateral triangular extensions, as shown in the diagram. What is the ratio of the area of the extensions to the area of the original hexagon?

1:11:1

6:56:5

3:23:2

2:12:1

3:13:1

Answer: A
Solution:

Note that we can split the hexagon into 66 congruent equilateral triangles as follows.

Since each of them share an edge with an exterior triangle, all the triangles are congruent. Therefore, the ratio of areas is 1:1.1:1.

Thus, A is the correct answer.

13.

Sets AA and B,B, shown in the Venn diagram, have the same number of elements. Their union has 20072007 elements and their intersection has 10011001 elements. Find the number of elements in A.A.

503503

10061006

15041504

15071507

15101510

Answer: C
Solution:

Let xx be the number of elements in each AA and B.B. Also let yy be the number of elements in their intersection.

The conditions give us that 2xy=2007 2x - y = 2007 and y=1001. y = 1001. Plugging yy into the first equation yields 2x1001=20072x=3008x=1504. \begin{align*} 2x - 1001 &= 2007 \\ 2x &= 3008 \\ x &= 1504. \end{align*}

Thus, C is the correct answer.

14.

The base of isosceles ABC\triangle ABC is 2424 and its area is 60.60. What is the length of one of the congruent sides?

55

88

1313

1414

1818

Answer: C
Solution:

Construct BD\overline{BD} as the altitude from BB to AC.\overline{AC}.

Then 60=12BD24, 60 = \dfrac{1}{2} \cdot BD \cdot 24, which gives us that BD=5.BD = 5.

From this, we apply the Pythagorean Theorem on ABD:\triangle ABD: AB2=52+122=169=132. AB^2 = 5^2 + 12^2 = 169 = 13^2. This gives us that AB=13.AB = 13.

Thus, C is the correct answer.

15.

Let a,ba, b and cc be numbers with 0<a<b<c.0 \lt a \lt b \lt c. Which of the following is impossible?

a+c<ba + c \lt b

ab<ca \cdot b \lt c

a+b<ca + b \lt c

ac<ba \cdot c \lt b

bc=a\dfrac{b}{c} = a

Answer: A
Solution:

We know that b<cb \lt c and 0<a.0 \lt a. Adding these two inequalities together yields b<c+a. b \lt c + a. This shows that A is impossible, and therefore the right answer.

To ensure that this is correct, we can show that the other options are possible.

B and C : a=1,a = 1, b=2,b = 2, and c=4c = 4

D : a=13,a = \dfrac{1}{3}, b=12,b = \dfrac{1}{2}, and c=1c = 1

E : a=12,a = \dfrac{1}{2}, b=1,b = 1, and c=2c = 2

Thus, A is the correct answer.

16.

Amanda Reckonwith draws five circles with radii 1,2,3,41, 2, 3, 4 and 5.5. Then for each circle she plots the point (C,A),(C, A), where CC is its circumference and AA is its area. Which of the following could be her graph?

Answer: A
Solution:

The circumferences of circles with radii 11 through 55 are 2π,4π,6π,8π,10π2\pi,4\pi,6\pi,8\pi,10\pi. Their respective areas are π,4π,9π,16π,25π\pi,4\pi,9\pi,16\pi,25\pi.

As CC increases by equal amounts, AA increases more and more quickly, so the points form an increasing quadratic pattern. Only graph A has that shape.

Thus, A is the correct answer.

17.

A mixture of 3030 liters of paint is 25%25\% red tint, 30%30\% yellow tint and 45%45\% water. Five liters of yellow tint are added to the original mixture. What is the percent of yellow tint in the new mixture?

2525

3535

4040

4545

5050

Answer: C
Solution:

The amount of yellow tint in the original mixture is .3×30=9.3 \times 30 = 9 liters. Adding 55 liters results in a total of 1414 liters of yellow tint.

The new mixture has a total of 3535 liters, so the percent of yellow tint is 1001435=10025=40%. 100 \cdot \dfrac{14}{35} = 100 \cdot \dfrac{2}{5} = 40 \%.

Thus, C is the correct answer.

18.

The product of the two 9999-digit numbers 303,030,303,,030,303303,030,303,\ldots,030,303 and 505,050,505,,050,505505,050,505,\ldots,050,505 has thousands digit AA and units digit B.B. What is the sum of AA and B?B?

33

55

66

88

1010

Answer: D
Solution:

Only the last four digits can affect the thousands digit and units digit of the product. The two numbers end in 03030303 and 05050505, so it is enough to compute 303505303\cdot 505.

Since 303505=153015303\cdot 505=153015, the last four digits of the full product are 30153015.

This gives A=3A=3 and B=5B=5, so A+B=8A+B=8.

Thus, D is the correct answer.

19.

Pick two consecutive positive integers whose sum is less than 100.100. Square both of those integers and then find the difference of the squares. Which of the following could be the difference?

22

6464

7979

9696

131131

Answer: C
Solution:

Let the consecutive positive integers be xx and x+1x+1. Their sum is 2x+12x+1, which is less than 100100.

The difference of their squares is (x+1)2x2=(x+1+x)(x+1x)=2x+1.(x+1)^2-x^2=(x+1+x)(x+1-x)=2x+1.

So the difference must be an odd number less than 100100. Among the choices, 7979 is the only possibility, and it occurs for 3939 and 4040.

Thus, C is the correct answer.

20.

Before district play, the Unicorns had won 45%45 \% of their basketball games. During district play, they won six more games and lost two, to finish the season having won half their games. How many games did the Unicorns play in all?

4848

5050

5252

5454

6060

Answer: A
Solution:

Let xx be the number of games the Unicorns had played before district play. Then they had won 0.45x0.45x games before district play and 0.45x+60.45x+6 games after district play.

In total, they played x+8x+8 games. The problem statement tells us that 0.45x+6=x+82.0.45x+6=\dfrac{x+8}{2}.

Solving gives 0.45x+6=0.5x+40.45x+6=0.5x+4, so 2=0.05x2=0.05x, and x=40x=40. Therefore, the Unicorns played 40+8=4840+8=48 games in all.

Thus, A is the correct answer.

21.

Two cards are dealt from a deck of four red cards labeled A,A, B,B, C,C, DD and four green cards labeled A,A, B,B, C,C, D.D. A winning pair is two of the same color or two of the same letter. What is the probability of drawing a winning pair?

27\dfrac{2}{7}

38\dfrac{3}{8}

12\dfrac{1}{2}

47\dfrac{4}{7}

58\dfrac{5}{8}

Answer: D
Solution:

After drawing the first card, there are 77 left. 33 of them have the same color, and 11 of them has the same letter. Therefore, there are 44 out of 77 possibilities that result in a winning pair.

The probability of drawing a pair is then 47.\dfrac{4}{7}.

Thus, D is the correct answer.

22.

A lemming sits at a corner of a square with side length 1010 meters. The lemming runs 6.26.2 meters along a diagonal toward the opposite corner. It stops, makes a 9090^{\circ} right turn and runs 22 more meters. A scientist measures the shortest distance between the lemming and each side of the square. What is the average of these four distances in meters?

22

4.54.5

55

6.26.2

77

Answer: C
Solution:

Based on the lengths given in the problem, the lemming is still in the square after it stops.

Since the lemming is still in the square, the sum of the distances to the horizontal sides is 1010 meters and the same for the vertical sides. Therefore, the 44 distances sum to 2020 meters, making the average 20÷4=520 \div 4 = 5 meters.

Thus, C is the correct answer.

23.

What is the area of the shaded pinwheel shown in the 5×55 \times 5 grid?

44

66

88

1010

1212

Answer: B
Solution:

We can find the area of the shaded part by subtracting the area of the unshaded part from the whole area.

There are 44 squares with side length 1,1, contributing 11 each to the unshaded area, for a total of 4.4.

There are also 44 triangles with base 33 and height 52.\dfrac{5}{2}. This contributes a total area of 412352=15. 4 \cdot \dfrac{1}{2} \cdot 3 \cdot \dfrac{5}{2} = 15.

The total unshaded area is therefore 4+15=19.4 + 15 = 19.

The total area is 52=25,5^2 = 25, and subtracting the unshaded area yields 2519=625 - 19 = 6 as the area of the shaded region.

Thus, B is the correct answer.

24.

A bag contains four pieces of paper, each labeled with one of the digits 1,1, 2,2, 33 or 4,4, with no repeats. Three of these pieces are drawn, one at a time without replacement, to construct a three-digit number. What is the probability that the three-digit number is a multiple of 3?3?

14\dfrac{1}{4}

13\dfrac{1}{3}

12\dfrac{1}{2}

23\dfrac{2}{3}

34\dfrac{3}{4}

Answer: C
Solution:

Recall that a number is divisible by 33 if the sum of its digits is divisible by 33.

The only triples of distinct digits from 1,2,3,41,2,3,4 whose sum is divisible by 33 are (1,2,3)(1,2,3) and (2,3,4)(2,3,4).

This means the constructed number is a multiple of 33 exactly when either 11 or 44 is left in the bag.

Each of these events has probability 14\dfrac{1}{4}, for a total probability of 214=122\cdot\dfrac{1}{4}=\dfrac{1}{2}.

Thus, C is the correct answer.

25.

On the dart board shown in the figure below, the outer circle has radius 66 and the inner circle has radius 3.3. Three radii divide each circle into three congruent regions, with point values shown. The probability that a dart will hit a given region is proportional to the area of the region. When two darts hit this board, the score is the sum of the point values of the regions hit. What is the probability that the score is odd?

1736\dfrac{17}{36}

3572\dfrac{35}{72}

12\dfrac{1}{2}

3772\dfrac{37}{72}

1936\dfrac{19}{36}

Answer: B
Solution:

The area of the outer circle is 62π=36π,6^2\pi = 36\pi, and the area of the inner circle is 32π=9π.3^2\pi = 9\pi. Therefore, the area of the outer ring is 36π9π=27π.36\pi - 9\pi = 27\pi.

This means that the probability of hitting an outer segment is 9π36π=14.\dfrac{9\pi}{36\pi} = \dfrac{1}{4}. Similarly, the probability of hitting an inner segment is 3π36π=112.\dfrac{3\pi}{36\pi} = \dfrac{1}{12}.

Summing over all possibilities, the probability of hitting a 11 is 112+214=712. \dfrac{1}{12} + 2 \cdot \dfrac{1}{4} = \dfrac{7}{12}. Similarly, the probability of hitting a 22 is 2112+14=512. 2 \cdot \dfrac{1}{12} + \dfrac{1}{4} = \dfrac{5}{12}.

The only way to get an odd score is to hit one 11 and one 2.2.

The probability of hitting these numbers in either order is 2512712=3572. 2 \cdot \dfrac{5}{12} \cdot \dfrac{7}{12} = \dfrac{35}{72}.

Thus, B is the correct answer.