2013 AMC 10A 考试答案

Scroll down to view professionally written solutions curated by LIVE by Po-Shen Loh, print PDF solutions, view answer key, or:

Try Exam

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).

Want to learn professionally through interactive video classes?

1.

A taxi ride costs $1.50\$1.50 plus $0.25\$0.25 per mile traveled. How much does a 55-mile taxi ride cost?

$2.25\$2.25

$2.50\$2.50

$2.75\$2.75

$3.00\$3.00

$3.75\$3.75

Solution:

The mileage charge is $0.255=$1.25\$0.25\cdot 5=\$1.25.

Adding the fixed charge gives $1.50+$1.25=$2.75\$1.50+\$1.25=\$2.75.

Thus, C is the correct answer.

2.

Alice is making a batch of cookies and needs 2122\frac{1}{2} cups of sugar. Unfortunately, her measuring cup holds only 14\frac{1}{4} cup of sugar. How many times must she fill that cup to get the correct amount of sugar?

88

1010

1212

1616

2020

Solution:

Alice needs 212=522\frac12=\frac52 cups of sugar.

Each full measuring cup gives 14\frac14 cup, so the number of fillings is 52÷14=524=10\frac52\div\frac14=\frac52\cdot4=10.

Thus, B is the correct answer.

3.

Square ABCDABCD has side length 10.10. Point EE is on BC,\overline{BC}, and the area of ABE\triangle ABE is 40.40. What is BE?BE?

44

55

66

77

88

Solution:

We have by the formula for the area of a triangle that 40=1210BE. 40 = \dfrac{1}{2} \cdot 10 \cdot BE. This gives us 40=5BE 40 = 5BE BE=8. BE = 8. Thus, E is the correct answer.

4.

A softball team played ten games, scoring 1,1, 2,2, 3,3, 4,4, 5,5, 6,6, 7,7, 8,8, 9,9, and 1010 runs. They lost by one run in exactly five games. In each of their other games, they scored twice as many runs as their opponent. How many total runs did their opponents score?

3535

4040

4545

5050

5555

Solution:

Note that if they scored twice as many runs as their opponents, then they scored an even number of runs.

This means in the games where they scored 2,4,6,8,2, 4, 6, 8, and 1010 runs, their opponents scored 1,2,3,4,1, 2, 3, 4, and 55 runs respectively.

This sums to 1+2+3+4+5=15. 1 + 2 + 3 + 4 + 5 = 15.

In the other games, their opponents scored 2,4,6,8,2, 4, 6, 8, and 1010 runs.

This sums to 2+4+6+8+10=30. 2 + 4 + 6 + 8 + 10 = 30.

The total number of runs is then 15+30=45. 15 + 30 = 45. Thus, C is the correct answer.

5.

Tom, Dorothy, and Sammy went on a vacation and agreed to split the costs evenly. During their trip Tom paid $105,\$105, Dorothy paid $125,\$125, and Sammy paid $175.\$175. In order to share costs equally, Tom gave Sammy tt dollars, and Dorothy gave Sammy dd dollars. What is td?t-d?

1515

2020

2525

3030

3535

Solution:

The total cost was $105+$125+$175=$405\$105+\$125+\$175=\$405, so each person's share was $405÷3=$135\$405\div3=\$135.

Tom paid $30\$30 less than his share, and Dorothy paid $10\$10 less than her share, so t=30t=30 and d=10d=10.

Therefore td=3010=20t-d=30-10=20.

Thus, B is the correct answer.

6.

Joey and his five brothers are ages 3,3, 5,5, 7,7, 9,9, 11,11, and 13.13. One afternoon two of his brothers whose ages sum to 1616 went to the movies, two brothers younger than 1010 went to play baseball, and Joey and the 55-year-old stayed home. How old is Joey?

33

77

99

1111

1313

Solution:

The age pairs that sum to 1616 are (3,13)(3,13), (5,11)(5,11), and (7,9)(7,9).

The 55-year-old stayed home, so the movie pair cannot be (5,11)(5,11). If 77 and 99 went to the movies, then the only remaining brother younger than 1010 would be the 33-year-old, not enough for baseball.

Thus the movie pair was (3,13)(3,13), the baseball pair was (7,9)(7,9), and Joey is the remaining brother, age 1111.

Thus, D is the correct answer.

7.

A student must choose a program of four courses from a menu of courses consisting of English, Algebra, Geometry, History, Art, and Latin. This program must contain English and at least one mathematics course. In how many ways can this program be chosen?

66

88

99

1212

1616

Solution:

English is required, so choose the other 33 courses from the 55 courses Algebra, Geometry, History, Art, and Latin.

There are (53)=10\binom53=10 such choices, but one of them, History-Art-Latin, contains no mathematics course.

Therefore the number of valid programs is 101=910-1=9.

Thus, C is the correct answer.

8.

What is the value of 22014+220122201422012?\dfrac{2^{2014}+2^{2012}}{2^{2014}-2^{2012}} ?

1-1

11

53\dfrac{5}{3}

20132013

240242^{4024}

Solution:

Factoring out a 22012,2^{2012}, we get: 22012(22+1)22012(221)=53. \dfrac{2^{2012}(2^2 + 1)}{2^{2012}(2^2 - 1)} = \dfrac{5}{3}.

Thus, C is the correct answer.

9.

In a recent basketball game, Shenille attempted only three-point shots and two-point shots. She was successful on 20%20\% of her three-point shots and 30%30\% of her two-point shots. Shenille attempted 3030 shots. How many points did she score?

1212

1818

2424

3030

3636

Solution:

Let xx be the number of two-point shots and yy be the number of three-point shots.

Then, Shenille scores .3x.3x two-points shots and .2y.2y three-point shots, for a total score of 2.3x+3.2y=.6x+.6y. 2 \cdot .3x + 3 \cdot .2y = .6x + .6y.

We know that x+y=30 x + y = 30 .6(x+y)=18. .6(x + y) = 18.

Thus, B is the correct answer.

10.

A flower bouquet contains pink roses, red roses, pink carnations, and red carnations. One third of the pink flowers are roses, three fourths of the red flowers are carnations, and six tenths of the flowers are pink. What percent of the flowers are carnations?

1515

3030

4040

6060

7070

Solution:

Let the total number of flowers be x.x. There are .6x.6x pink flowers and .4x.4x red flowers.

Then there are 13.6x=.2x \dfrac{1}{3} \cdot .6x = .2x pink roses, which means there are .4x.4x pink carnations.

There are also 34.4x=.3x \dfrac{3}{4} \cdot .4x = .3x red carnations. This means there are .3x+.4x=.7x.3x + .4x = .7x carnations. This is 70%70\% of the total flowers.

Thus, E is the correct answer.

11.

A student council must select a two-person welcoming committee and a three-person planning committee from among its members. There are exactly 1010 ways to select a two-person team for the welcoming committee. It is possible for students to serve on both committees. In how many different ways can a three-person planning committee be selected?

1010

1212

1515

1818

2525

Solution:

Let xx be the number of students. Then the number of ways to pick a two-person committee is (x2)=x(x1)2. \binom{x}{2} = \dfrac{x(x - 1)}{2}. We know that this equals 10,10, so x2x=20 x^2 - x = 20 x2x20=0. x^2 - x - 20 = 0. Factoring yields (x5)(x+4)=0 (x - 5)(x + 4) = 0 x=5, x = 5, since there cannot be a negative number of students.

Then, the number of ways to pick a 33-person committee is (53)=(52)=10. \binom{5}{3} = \binom{5}{2} = 10.

Thus, A is the correct answer.

12.

In ABC,\triangle ABC, AB=AC=28AB=AC=28 and BC=20.BC=20. Points D,E,D,E, and FF are on sides AB,\overline{AB}, BC,\overline{BC}, and AC,\overline{AC}, respectively, such that DE\overline{DE} and EF\overline{EF} are parallel to AC\overline{AC} and AB,\overline{AB}, respectively. What is the perimeter of parallelogram ADEF?ADEF?

4848

5252

5656

6060

7272

Solution:

Note that DBEABC\triangle DBE \sim \triangle ABC and FECABC\triangle FEC \sim \triangle ABC due to the parallel lines.

This tells us that DB=DEDB = DE and FE=FC.FE = FC. We have that the perimeter of ADEFADEF is AD+DE+EF+AF AD + DE + EF + AF =AD+DB+FC+AF= AD + DB + FC + AF =AB+AC = AB + AC =56.= 56.

Thus, C is the correct answer.

13.

How many three-digit numbers are not divisible by 5,5, have digits that sum to less than 20,20, and have the first digit equal to the third digit?

5252

6060

6666

6868

7070

Solution:

Note that for the number to not be divisible by 5,5, the units digits cannot be either 00 or 5.5.

Let xx be the hundreds and units digit and yy be the tens digit. Then we want 2x+y<20. 2x + y \lt 20. Casing on the 99 options of x,x, we get:

If xx is 1,2,3,1, 2, 3, or 4,4, then yy can be anything since y<10.y \lt 10.

If x=6,x = 6, then y<8,y \lt 8, which gives us 88 solutions.

If x=7,x = 7, then y<6,y \lt 6, which gives us 66 solutions.

If x=8,x = 8, then y<4,y \lt 4, which gives us 44 solutions.

If x=9,x = 9, then y<2,y \lt 2, which gives us 22 solutions.

This gives us a total of 410+8+6+4+2=60 4 \cdot 10 + 8 + 6 + 4 + 2 = 60 solutions.

Thus, B is the correct solution.

14.

A solid cube of side length 11 is removed from each corner of a solid cube of side length 3.3. How many edges does the remaining solid have?

3636

6060

7272

8484

108108

Solution:

Removing the cubes does not remove any edges from the original cube. It only adds edges.

After removing each cube, we can see that 99 extra edges are added to the solid.

88 cubes are removed, which means 89=728 \cdot 9 = 72 edges are added to the original 1212 edges, for a total of 72+12=8472 + 12 = 84 edges.

Thus, D is the correct answer.

15.

Two sides of a triangle have lengths 1010 and 15.15. The length of the altitude to the third side is the average of the lengths of the altitudes to the two given sides. How long is the third side?

66

88

99

1212

1818

Solution:

Let h1h_1 be the length of the altitude to the side of length 1010 and similarly define h2h_2 for the other given side.

We have that 10h1=15h2 10h_1 = 15h_2 h1=32h2. h_1 = \dfrac{3}{2}h_2.

The third altitude is the average of the other two, which makes its length h2+32h22=54h2. \dfrac{h_2 + \frac{3}{2}h_2}{2} = \dfrac{5}{4}h_2.

Let the third side have length x.x. Then 54h2x=15h2 \dfrac{5}{4}h_2x = 15h_2 x=12. x = 12.

Thus, D is the correct answer.

16.

A triangle with vertices (6,5),(6, 5), (8,3),(8, -3), and (9,1)(9, 1) is reflected about the line x=8x = 8 to create a second triangle. What is the area of the union of the two triangles?

99

283\dfrac{28}{3}

1010

313\dfrac{31}{3}

323\dfrac{32}{3}

Solution:

The reflected triangle has vertices (7,1)(7,1), (8,3)(8,-3), and (10,5)(10,5).

The line through (6,5)(6,5) and (9,1)(9,1) is y=43x+13y=-\frac43x+13, so it meets x=8x=8 at y=73y=\frac73. By symmetry, the union is two congruent triangles with vertical base from (8,3)(8,-3) to (8,73)(8,\frac73).

That base has length 73+3=163\frac73+3=\frac{16}{3}, and each triangle has horizontal height 22. Hence the union area is 2(121632)=3232\left(\frac12\cdot\frac{16}{3}\cdot2\right)=\frac{32}{3}.

Thus, E is the correct answer.

17.

Daphne is visited periodically by her three best friends: Alice, Beatrix, and Claire. Alice visits every third day, Beatrix visits every fourth day, and Claire visits every fifth day.

All three friends visited Daphne yesterday. How many days of the next 365365-day period will exactly two friends visit her?

4848

5454

6060

6666

7272

Solution:

Pairwise visits occur every lcm(3,4)=12\operatorname{lcm}(3,4)=12, lcm(3,5)=15\operatorname{lcm}(3,5)=15, and lcm(4,5)=20\operatorname{lcm}(4,5)=20 days.

In the next 365365 days, these pair counts are 36512=30\left\lfloor\frac{365}{12}\right\rfloor=30, 36515=24\left\lfloor\frac{365}{15}\right\rfloor=24, and 36520=18\left\lfloor\frac{365}{20}\right\rfloor=18.

All three visit every 6060 days, which happens 36560=6\left\lfloor\frac{365}{60}\right\rfloor=6 times. Subtracting those days from each pair count gives (306)+(246)+(186)=54(30-6)+(24-6)+(18-6)=54.

Thus, B is the correct answer.

18.

Let points A=(0,0), B=(1,2),A = (0, 0), ~B = (1, 2), C=(3,3), D=(4,0).C=(3, 3),~ D = (4, 0). Quadrilateral ABCDABCD is cut into equal area pieces by a line passing through A.A. This line intersects CD\overline{CD} at point (pq,rs),\left(\dfrac{p}{q}, \dfrac{r}{s}\right), where these fractions are in lowest terms. What is p+q+r+s?p+q+r+s?

5454

5858

6262

7070

7575

Solution:

Let the cutting line meet CD\overline{CD} at GG. Drop perpendiculars from BB, CC, and GG to the xx-axis as in the diagram.

The areas of ABF\triangle ABF, trapezoid BCEFBCEF, and CDE\triangle CDE are 11, 55, and 32\frac32, respectively, so [ABCD]=152[ABCD]=\frac{15}{2}.

Thus ADG\triangle ADG has area 154\frac{15}{4}. Since AD=4AD=4, the height of GG is 158\frac{15}{8}.

The line CDCD has equation y=3x+12y=-3x+12, so 158=3x+12\frac{15}{8}=-3x+12, giving x=278x=\frac{27}{8}. Therefore p+q+r+s=27+8+15+8=58p+q+r+s=27+8+15+8=58.

Thus, B is the correct answer.

19.

In base 10,10, the number 20132013 ends in the digit 3.3. In base 9,9, on the other hand, the same number is written as (2676)9(2676)_9 and ends in the digit 6.6. For how many positive integers bb does the base-bb-representation of 20132013 end in the digit 3?3?

66

99

1313

1616

1818

Solution:

Note that the units digit of represents the remainder when the number is divided by the base.

The question then boils down to finding all numbers, b,b, such that 20132013 leaves a remainder of 33 when divided by b.b.

This means that bb must divide 2010.2010. Also note that b4,b \geq 4, since otherwise the remainder cannot be 3.3.

The prime factorization of 20102010 is 2010=23567. 2010 = 2 \cdot 3 \cdot 5 \cdot 67. Then, 20102010 has (1+1)4=24=16 (1 + 1)^4 = 2^4 = 16 factors. It has 33 factors less than 4,4, namely 1,2,1, 2, and 3.3. This means there are 163=1316 - 3 = 13 valid values for b.b.

Thus, C is the correct answer.

20.

A unit square is rotated 4545^\circ about its center. What is the area of the region swept out by the interior of the square?

122+π41 - \dfrac{\sqrt2}{2} + \dfrac{\pi}{4}

12+π4\dfrac{1}{2} + \dfrac{\pi}{4}

22+π42 - \sqrt2 + \dfrac{\pi}{4}

22+π4\dfrac{\sqrt2}{2} + \dfrac{\pi}{4}

1+24+π81 + \dfrac{\sqrt2}{4} + \dfrac{\pi}{8}

Solution:

Consider one quarter of the swept region and multiply its area by 44.

The sector has angle 4545^\circ and radius 22\frac{\sqrt2}{2}, so its area is 18π(22)2=π16\frac18\pi\left(\frac{\sqrt2}{2}\right)^2=\frac{\pi}{16}.

The two right-triangle pieces in that quarter have areas 121212=18\frac12\cdot\frac12\cdot\frac12=\frac18 and 12(212)2\frac12\left(\frac{\sqrt2-1}{2}\right)^2.

Multiplying the quarter-area sum by 44 gives 22+π42-\sqrt2+\frac{\pi}{4}.

Thus, C is the correct answer.

21.

A group of 1212 pirates agree to divide a treasure chest of gold coins among themselves as follows. The kthk^{\text{th}} pirate to take a share takes k12\dfrac{k}{12} of the coins that remain in the chest. The number of coins initially in the chest is the smallest number for which this arrangement will allow each pirate to receive a positive whole number of coins. How many coins does the 12th12^{\text{th}} pirate receive?

720720

12961296

17281728

19251925

38503850

Solution:

Work backward. If nn coins remain for the 1212th pirate, then before pirate kk took a share, the chest had 1212k\frac{12}{12-k} times as many coins as it had afterward.

Therefore the initial number of coins is n121111!n\cdot\frac{12^{11}}{11!}.

Since 121111!=214375711\frac{12^{11}}{11!}=\frac{2^{14}3^7}{5\cdot7\cdot11}, the smallest nn that makes the initial number an integer is 52711=19255^2\cdot7\cdot11=1925.

Thus, the 1212th pirate receives 19251925 coins, and D is the correct answer.

22.

Six spheres of radius 11 are positioned so that their centers are at the vertices of a regular hexagon of side length 2.2. The six spheres are internally tangent to a larger sphere whose center is the center of the hexagon. An eighth sphere is externally tangent to the six smaller spheres and internally tangent to the larger sphere. What is the radius of this eighth sphere?

2\sqrt2

32\dfrac{3}{2}

53\dfrac{5}{3}

3\sqrt3

22

Solution:

The centers of the six radius-11 spheres form a regular hexagon of side length 22, so each is 22 units from the large sphere's center. Hence the large sphere has radius 33.

Let the eighth sphere have radius rr, and let its center be distance xx from the large sphere's center. Internal tangency gives x+r=3x+r=3, so x=3rx=3-r.

Using the right triangle between the large center, a small-sphere center, and the eighth-sphere center, (r+1)2=22+(3r)2(r+1)^2=2^2+(3-r)^2. Thus 2r+1=136r2r+1=13-6r, so r=32r=\frac32.

Thus, B is the correct answer.

23.

In ABC,\triangle ABC, AB=86,AB = 86, and AC=97.AC=97. A circle with center AA and radius ABAB intersects BC\overline{BC} at points BB and X.X. Moreover BX\overline{BX} and CX\overline{CX} have integer lengths. What is BC?BC?

1111

2828

3333

6161

7272

Solution:

By power of a point from CC, CBCX=AC2AB2=972862CB\cdot CX=AC^2-AB^2=97^2-86^2.

This equals (9786)(97+86)=11183=2013=31161(97-86)(97+86)=11\cdot183=2013=3\cdot11\cdot61.

Both CXCX and BXBX are integers, so BC=BX+CXBC=BX+CX is an integer factor paired with CXCX. Also CX<BC<86+97=183CX<BC<86+97=183, so the only possible pair is CX=33CX=33, BC=61BC=61.

Thus, D is the correct answer.

24.

Central High School is competing against Northern High School in a backgammon match. Each school has three players, and the contest rules require that each player play two games against each of the other school's players. The match takes place in six rounds, with three games played simultaneously in each round. In how many different ways can the match be scheduled?

540540

600600

720720

810810

900900

Solution:

Label Central's players A,B,CA,B,C and Northern's players X,Y,ZX,Y,Z.

Player AA's six-round opponent string contains two each of X,Y,ZX,Y,Z, so it can be chosen in 6!2!2!2!=90\frac{6!}{2!2!2!}=90 ways.

For a fixed AA-string, say XXYYZZXXYYZZ, player BB's string must also contain two each of X,Y,ZX,Y,Z and cannot match AA's opponent in any position.

If BB's first two entries are Y,ZY,Z in either order, then the middle two entries must be X,ZX,Z in either order and the last two must be X,YX,Y in either order, giving 23=82^3=8 strings. The two remaining possibilities are YYZZXXYYZZXX and ZZXXYYZZXXYY, for 1010 total BB-strings.

Once AA and BB are scheduled, CC's schedule is forced. Hence there are 9010=90090\cdot10=900 schedules.

Thus, E is the correct answer.

25.

All 2020 diagonals are drawn in a regular octagon. At how many distinct points in the interior of the octagon (not on the boundary) do two or more diagonals intersect?

4949

6565

7070

9696

128128

Solution:

If no three diagonals were concurrent, each choice of 44 vertices would determine one interior intersection, giving (84)=70\binom84=70.

The 44 long diagonals through opposite vertices all meet at the center, so the center was counted (42)=6\binom42=6 times and should be counted once. Subtract 55.

There are also 88 symmetric points where 33 diagonals meet. Each was counted (32)=3\binom32=3 times and should be counted once, so subtract 8(31)=168(3-1)=16.

The number of distinct interior intersection points is 70516=4970-5-16=49.

Thus, A is the correct answer.