2011 AMC 8 考试答案

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All of the real AMC 8 and AMC 10 problems in our complete solution collection are used with official permission of the Mathematical Association of America (MAA).

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1.

Margie bought 33 apples at a cost of 5050 cents per apple. She paid with a 55-dollar bill. How much change did Margie receive?

$1.50\$ 1.50

$2.00\$ 2.00

$2.50\$ 2.50

$3.00\$ 3.00

$3.50\$ 3.50

Solution:

The apples costed a total of 350=1503 \cdot 50 = 150 cents, which equals 1.50$.1.50 \$. This means that Margie received 51.5=3.55 - 1.5 = 3.5 dollars in change.

Thus, E is the correct answer.

2.

Karl's rectangular vegetable garden is 2020 feet by 4545 feet, and Makenna's is 2525 feet by 4040 feet. Whose garden is larger in area?

Karl’s garden is larger by 100100 square feet.

Karl’s garden is larger by 2525 square feet.

The gardens are the same size.

Makenna’s garden is larger by 2525 square feet.

Makenna’s garden is larger by 100100 square feet.

Solution:

The area of Karl's garden is 20ft45ft=900ft2.20 \text{ft} \cdot 45 \text{ft} = 900 \text{ft}^2. The area of Makenna's garden is 25ft40ft=1000ft2.25 \text{ft} \cdot 40 \text{ft} = 1000 \text{ft}^2.

The difference of these area is 1000ft2900ft2=100ft2.1000 \text{ft}^2 - 900 \text{ft}^2 = 100 \text{ft}^2. Therefore, Makenna's garden is 100ft2100 \text{ft}^2 larger than Karl's.

Thus, E is the correct answer.

3.

Extend the square pattern of 88 colored and 1717 uncolored square tiles by attaching a border of colored tiles around the square. What is the ratio of colored tiles to uncolored tiles in the extended pattern?

8:178:17

25:4925:49

36:2536:25

32:1732:17

36:1736:17

Solution:

In the extended figure, there are 3232 colored tiles and 1717 uncolored tiles. Therefore, the ratio of colored tiles to uncolored tiles is 32:17.32:17.

Thus, D is the correct answer.

4.

Here is a list of the numbers of fish that Tyler caught in nine outings last summer: 2,0,1,3,0,3,3,1,2.2,0,1,3,0,3,3,1,2. Which statement about the mean, median, and mode is true?

median < mean < mode

mean < mode < median

mean < median < mode

median < mode < mean

mode < median < mean

Solution:

To find these values more easily, we can get the following ordered list: 0,0,1,1,2,2,3,3,3. 0, 0, 1, 1, 2, 2, 3, 3, 3.

From this, we see that the mode is 3,3, the median is 2,2, and the mean is 15/9=5/3.15 / 9 = 5 / 3.

Since 53<2<3,\dfrac{5}{3} < 2 < 3, we get that:

mean << median << mode.

Thus, C is the correct answer.

5.

What time was it 20112011 minutes after midnight on January 1,1, 2011?2011?

January 11 at 9:319:31 PM

January 11 at 11:5111:51 PM

January 22 at 3:113:11 AM

January 22 at 9:319:31 AM

January 22 at 6:016:01 PM

Solution:

The remainder when 20112011 is divided by 6060 is 31.31. This means that 2011=6033+31,2011 = 60 \cdot 33 + 31, which means that 20112011 minutes is the same as 3333 hours and 3131 minutes.

2424 hours takes us to January 2,2, so we get that we are 99 hours and 3131 minutes into January 2.2.

Thus, D is the correct answer.

6.

In a town of 351351 adults, every adult owns a car, motorcycle, or both. If 331331 adults own cars and 4545 adults own motorcycles, how many of the car owners do not own a motorcycle?

2020

2525

4545

306306

351351

Solution:

We know that 4545 people own motorcycles, so 35145=306351 - 45 = 306 people do not own motorcycles.

Thus, D is the correct answer.

7.

Each of the following four large congruent squares is subdivided into combinations of congruent triangles or rectangles and is partially shaded. What percent of the total area is partially shaded?

121212 \dfrac{1}{2}

2020

2525

331333 \dfrac{1}{3}

371237 \dfrac{1}{2}

Solution:

The top left and the bottom right shaded regions are both a quarter of each square. The top right is one-eight, and the bottom left is three-eights. Their combined area is 14+14+18+38=1.\dfrac{1}{4} + \dfrac{1}{4} + \dfrac{1}{8} + \dfrac{3}{8} = 1.

Therefore, the shaded regions combined equal the area of one square, so they are 25%25 \% of the total area.

Thus, C is the correct answer.

8.

Bag A has three chips labeled 1,1, 3,3, and 5.5. Bag B has three chips labeled 2,2, 4,4, and 6.6. If one chip is drawn from each bag, how many different values are possible for the sum of the two numbers on the chips?

44

55

66

77

99

Solution:

We can create a table to look at all the possible outcomes and their respective sums.

From this, we can see that there are 55 distinct values that we can get.

Thus, B is the correct answer.

9.

Carmen takes a long bike ride on a hilly highway. The graph indicates the miles traveled during the time of her ride. What is Carmen's average speed for her entire ride in miles per hour?

22

2.52.5

44

4.54.5

55

Solution:

Carmen travels 3535 miles in 77 hours, so her average speed is 35/7=535 / 7 = 5 miles per hour.

Thus, E is the correct answer.

10.

The taxi fare in Gotham City is $ 2.40 for the first 12\dfrac{1}{2} mile and additional mileage charged at the rate $ 0.20 for each additional 0.10.1 mile. You plan to give the driver a $ 2 tip. How many miles can you ride for $ 10?

3.03.0

3.253.25

3.33.3

3.53.5

3.753.75

Solution:

There is a guaranteed $ 2 tip, so we can subtract that from the total, leaving $ 8. This is greater than $ 2.40, so we can subtract that and add 12\dfrac{1}{2} miles to the total distance.

We now have $ 5.60 to use for additional miles. $ 0.20 per 0.10.1 mile is the same as $ 2 for 11 mile. That means one can ride for 5.60/2=2.85.60 / 2 = 2.8 more miles with this much money. This leaves a total of 2.8+12=3.32.8 + \dfrac{1}{2} = 3.3 miles.

Thus, C is the correct answer.

11.

The graph shows the number of minutes studied by both Asha (black bar) and Sasha (grey bar) in one week. On the average, how many more minutes per day did Sasha study than Asha?

66

88

99

1010

1212

Solution:

We can calculate the difference in average minutes by looking at the differences per day.

Starting with Monday, the differences between Sasha and Asha are 10,10, 10,-10, 20,20, 30,30, and 20.-20. This is a total of 3030 minutes. Therefore, the average difference is 30÷5=6.30 \div 5 = 6.

Thus, A is the correct answer.

12.

Angie, Bridget, Carlos, and Diego are seated at random around a square table, one person to a side. What is the probability that Angie and Carlos are seated opposite each other?

14\dfrac{1}{4}

13\dfrac{1}{3}

12\dfrac{1}{2}

23\dfrac{2}{3}

34\dfrac{3}{4}

Solution:

Consider that Angie's seat is chosen. Carlos has an equal probability of being in any of the other 33 seats. Only one of them is opposite Angie, however. Therefore, the probability is 13.\dfrac{1}{3}.

Thus, B is the correct answer.

13.

Two congruent squares, ABCDABCD and PQRS,PQRS, have side length 15.15. They overlap to form the 1515 by 2525 rectangle AQRDAQRD shown. What percent of the area of rectangle AQRDAQRD is shaded?

1515

1818

2020

2424

2525

Solution:

We get that SC=DC+SRDR=15+1525=5.\begin{align*} SC &= DC + SR - DR \\ &= 15 + 15 - 25 \\ &= 5. \end{align*}

This means that the area of PBSCPBSC is 515=75.5 \cdot 15 = 75. The area of AQRDAQRD is 2515=375.25 \cdot 15 = 375.

75375=15,\dfrac{75}{375} = \dfrac{1}{5}, which is 20%.20 \%.

Thus, C is the correct answer.

14.

There are 270270 students at Colfax Middle School, where the ratio of boys to girls is 5:4.5 : 4. There are 180180 students at Winthrop Middle School, where the ratio of boys to girls is 4:5.4 : 5. The two schools hold a dance and all students from both schools attend. What fraction of the students at the dance are girls?

718\dfrac{7}{18}

715\dfrac{7}{15}

2245\dfrac{22}{45}

12\dfrac{1}{2}

2345\dfrac{23}{45}

Solution:

The total number of girls is 49270+59180=\dfrac{4}{9} \cdot 270 + \dfrac{5}{9} \cdot 180 = 120+100=220. 120 + 100 = 220.

There are 270+180=450270 + 180 = 450 students total, so the fraction of girls is 220450=2245.\dfrac{220}{450} = \dfrac{22}{45}.

Thus, C is the correct answer.

15.

How many digits are in the product 45510?4^5 \cdot 5^{10}?

88

99

1010

1111

1212

Solution:

To find the number of digits, we can try to express this number in terms of powers of 10.10.

We get that 45510=210510=1010.\begin{align*} 4^5 \cdot 5^{10} &= 2^{10} \cdot 5^{10} \\ &= 10^{10}. \end{align*}

This shows that the desired number is 11 followed by 1010 zeros, for a total of 1111 digits.

Thus, D is the correct answer.

16.

Let AA be the area of the triangle with sides of length 25,25,25, 25, and 30.30. Let BB be the area of the triangle with sides of length 25,25,25, 25, and 40.40. What is the relationship between AA and B?B?

A=916BA = \dfrac{9}{16}B

A=34BA = \dfrac{3}{4}B

A=BA = B

A=43BA = \dfrac{4}{3}B

A=169BA = \dfrac{16}{9}B

Solution:

Since these triangle are isosceles, we can drop altitudes to create two congruent right triangles as shown in the diagram.

Using the Pythagorean theorem, we get that altitude of the triangle with area AA equals 252152=20.\sqrt{25^2 - 15^2} = 20. Similarly, we get that the altitude of the triangle with area BB equals 252202=15.\sqrt{25^2 - 20^2} = 15.

With these altitudes, we can calculate the areas of the triangles. We get that A=122030=300. A = \dfrac{1}{2} \cdot 20 \cdot 30 = 300. Similarly, B=121540=300. B = \dfrac{1}{2} \cdot 15 \cdot 40 = 300.

Therefore, A=B.A = B.

Thus, C is the correct answer.

17.

Let w,w, x,x, y,y, and zz be whole numbers. If 2w3x5y7z=588,2^w \cdot 3^x \cdot 5^y \cdot 7^z = 588, then what does 2w+3x+5y+7z2w + 3x + 5y + 7z equal?

2121

2525

2727

3535

5656

Solution:

To find the desired exponents, note that all the bases are prime numbers. This means that finding the prime factorization will be helpful.

We get that 588=223172.588 = 2^2 \cdot 3^1 \cdot 7^2.

From this, it is clear that w=2,w = 2, x=1,x = 1, y=0,y = 0, and z=2z = 2 (y=0y = 0 since that makes the 5y5^y term equal 11).

Therefore, \begin{gather*} 2w + 3x + 5y + 7z \\ = 2 \cdot 2 + 3 \cdot 1 + 5 \cdot 0 + 7 \cdot 2 \\ = 21. \end{gather*}

Thus, A is the correct answer.

18.

A fair 66 sided die is rolled twice. What is the probability that the first number that comes up is greater than or equal to the second number?

16\dfrac{1}{6}

512\dfrac{5}{12}

12\dfrac{1}{2}

712\dfrac{7}{12}

56\dfrac{5}{6}

Solution:

There are 33 possible outcomes when rolling a die twice: the first number is greater than the second, both numbers are equal, or the first number is less than the second number. The first and third outcomes have the same probability since they are symmetric.

The second outcome has a 16\dfrac{1}{6} chance of happening, since the first number can be anything, and the second number must equal first number. The other two outcomes have a combined probability of 116=56.1 - \dfrac{1}{6} = \dfrac{5}{6}. This means that each outcome has a 56÷2=512\dfrac{5}{6} \div 2 = \dfrac{5}{12} chance of happening.

The desired probability is the first outcome plus the second outcome, for a total probability of 512+16=712.\dfrac{5}{12} + \dfrac{1}{6} = \dfrac{7}{12}.

Thus, D is the correct answer.

19.

How many rectangles are in this figure?

88

99

1010

1111

1212

Solution:

We can split the figure into these regions to make it easier to count the rectangles.

The rectangles in this figure are b,c,d,ab,bc,cd,cf,de,abcd,cdef,b, c, d, ab, bc, cd, cf, de, abcd, cdef, and bcfg.bcfg. These form 1111 rectangles.

Thus, D is the correct answer.

20.

Quadrilateral ABCDABCD is a trapezoid, AD=15,AD = 15, AB=50,AB = 50, BC=20,BC = 20, and the altitude is 12.12. What is the area of the trapezoid?

600600

650650

700700

750750

800800

Solution:

We can drop the following altitudes to more easily find the area.

We can use the Pythagorean to get that DE=152122=9 DE = \sqrt{15^2 - 12^2} = 9 and FC=202122=16. FC = \sqrt{20^2 - 12^2} = 16.

We also know that EF=AB=50, EF = AB = 50, so DC=DE+EF+FC=75. DC = DE + EF + FC = 75.

Then the area of ABCDABCD is 12(DC+50)12=6125 \dfrac{1}{2} \cdot (DC + 50) \cdot 12 = 6 \cdot 125 =750. = 750.

Thus, D is the correct answer.

21.

Students guess that Norb's age is 24,28,30,32,36,38,41,44,47,24, 28, 30, 32, 36, 38, 41, 44, 47, and 49.49. Norb says, "At least half of you guessed too low, two of you are off by one, and my age is a prime number." How old is Norb?

2929

3131

3737

4343

4848

Solution:

The first part of the statement means that Norb's age is greater than 36.36.

The second part means that Norb's age is either between 3636 and 3838 or between 4747 and 49.49.

Since 3737 is prime and 4848 is not, Norb's age is 37.37.

Thus, C is the correct answer.

22.

What is the tens digit of 72011?7^{2011}?

00

11

33

44

77

Solution:

Solution 1

To find the tens digit, we can simply find the tens digit when taking the number mod100. \mod 100. Since 74=24011mod100,7^4 = 2401 \equiv 1 \mod 100, then 7201174503737334343.7^{2011} \equiv 7^{4\cdot503} \cdot 7^3 \equiv 7^3 \equiv 343 \equiv 43.

Solution 2

If we look at the tens digits of powers of 7,7, we get 01,07,49,43,01,01, 07, 49, 43, 01, and from there the pattern repeats.

This means that the tens digits repeat in periods of 4.4. Since 20112011 leaves a remainder of 33 when divided by 4,4, its tens digit will be 4.4.

Thus, D is the correct answer.

23.

How many 44-digit positive integers have four different digits, where the leading digit is not zero, the integer is a multiple of 5,5, and 55 is the largest digit?

2424

4848

6060

8484

108108

Solution:

For a number to be divisible by 5,5, the units digit must be either 00 or 5.5.

If the units digit is 0,0, one of the other three digits must be 5.5. The remaining two digits must be chosen from {1,2,3,4}.\{1, 2, 3, 4\}. There are 66 ways to choose the pair, and there are 66 ways to arrange the three digits for a total of 66=366 \cdot 6 = 36 numbers.

If the units digit is 5,5, there are 44 ways to choose the thousands digit. There are 43=124 \cdot 3 = 12 ways to choose the other 22 digits. This leaves a total of 412=484 \cdot 12 = 48 numbers for this case.

Combining both cases, we get the total number of such integers is 36+48=84.36 + 48 = 84.

Thus, D is the correct answer.

24.

In how many ways can 1000110001 be written as the sum of two primes?

00

11

22

33

44

Solution:

For two numbers to add to an odd number, one of them must be odd and the other even. Thus only even prime is 2,2, so the other number is forced to be 9999.9999. 99999999 is not prime, however, so 1000110001 cannot be written as the sum of two primes.

Thus, A is the correct answer.

25.

A circle with radius 11 is inscribed in a square and circumscribed about another square as shown. Which fraction is closest to the ratio of the circle's shaded area to the area between the two squares?

12\dfrac{1}{2}

11

32\dfrac{3}{2}

22

52\dfrac{5}{2}

Solution:

The circle's shaded area is equal to the area of the circle minus the area of the smaller square. The side length of the inner square can be calculate using the Pythagorean Theorem to get 12+12=2.\sqrt{1^2 + 1^2} = \sqrt{2}.

Therefore, the area of the inner square is 22=2.\sqrt{2}^2 = 2. The area of the circle's shaded area is then 12π2=π2.1^2\pi - 2 = \pi - 2.

The area of the outside square is 22=4,2^2 = 4, so the area of the shaded area between the two squares is 42=2.4 - 2 = 2.

The desired fraction is π223.142212.\dfrac{\pi - 2}{2} \approx \dfrac{3.14 - 2}{2} \approx \dfrac{1}{2}.

Thus, A is the correct answer.