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AMC 10 2013 B

Question 1

What is ?

Solution

  
  2020-07-09 06:35:46

Question 2

Mr. Green measures his rectangular garden by walking two of the sides and finding that it is steps by steps. Each of Mr. Green's steps is feet long. Mr. Green expects a half a pound of potatoes per square foot from his garden. How many pounds of potatoes does Mr. Green expect from his garden?

Solution

  
  2020-07-09 06:35:46

Question 3

On a particular January day, the high temperature in Lincoln, Nebraska, was degrees higher than the low temperature, and the average of the high and the low temperatures was . In degrees, what was the low temperature in Lincoln that day?

Solution

  
  2020-07-09 06:35:46

Question 4

When counting from to , is the number counted. When counting backwards from to , is the number counted. What is ?

Solution

  
  2020-07-09 06:35:46

Question 5

Positive integers and are each less than . What is the smallest possible value for ?


Solution

  
  2020-07-09 06:35:46

Question 6

The average age of 33 fifth-graders is 11. The average age of 55 of their parents is 33. What is the average age of all of these parents and fifth-graders?

Solution

  
  2020-07-09 06:35:46

Question 7

Six points are equally spaced around a circle of radius 1. Three of these points are the vertices of a triangle that is neither equilateral nor isosceles. What is the area of this triangle?


Solution

  
  2020-07-09 06:35:46

Question 8

Ray's car averages 40 miles per gallon of gasoline, and Tom's car averages 10 miles per gallon of gasoline. Ray and Tom each drive the same number of miles. What is the cars' combined rate of miles per gallon of gasoline?


Solution

  
  2020-07-09 06:35:46

Question 9

Three positive integers are each greater than , have a product of , and are pairwise relatively prime. What is their sum?

Solution

  
  2020-07-09 06:35:46

Question 10

A basketball team's players were successful on 50% of their two-point shots and 40% of their three-point shots, which resulted in 54 points. They attempted 50% more two-point shots than three-point shots. How many three-point shots did they attempt?

Solution

  
  2020-07-09 06:35:46

Question 11

Real numbers and satisfy the equation . What is ?

Solution 1

  
SheenaY  2017-07-28 23:45:15

Solution 2

If we bring the x and y terms to the left side, we get (x-5)^2 + (y+3)^2 = 0. Squares of real numbers are nonnegative, so we need both (x-5)^2 and (y+3)^2 to be 0. This means x = 5 and y = -3. Therefore, x+y = 5 + (-3) = 2.

  
  2017-03-26 13:10:40

Question 12

Let be the set of sides and diagonals of a regular pentagon. A pair of elements of are selected at random without replacement. What is the probability that the two chosen segments have the same length?

Solution

In a regular pentagon, there are 5 sides with the same length, and 5 diagonals with the same length. Picking an element at random will leave 4 elements with the same length as the element picked, with 9 total elements remaining. Therefore, the probability is 4/9
  
  2017-01-07 21:10:29

Question 13

Jo and Blair take turns counting from to one more than the last number said by the other person. Jo starts by saying "", so Blair follows by saying "" . Jo then says "" , and so on. What is the 53rd number said?

Solution

  
  2020-07-09 06:35:46

Question 14

Define . Which of the following describes the set of points for which ?

Solution

x^2y-xy^2 = y^2x-yx^2. Then,

2x^2y-2xy^2=0.
2xy(x-y)=0.

Now, the solutions are obviously x=0, y=0, or x=y, which each correspond to a line. Thus, the answer is E
  
  2017-01-07 21:13:07

Question 15

A wire is cut into two pieces, one of length and the other of length . The piece of length is bent to form an equilateral triangle, and the piece of length is bent to form a regular hexagon. The triangle and the hexagon have equal area. What is ?

Solution

  
  2020-07-09 06:35:46

Question 16

In triangle , medians and intersect at , , , and . What is the area of ?

Solution

  
  2020-07-09 06:35:46

Question 17

Alex has red tokens and blue tokens. There is a booth where Alex can give two red tokens and receive in return a silver token and a blue token, and another booth where Alex can give three blue tokens and receive in return a silver token and a red token. Alex continues to exchange tokens until no more exchanges are possible. How many silver tokens will Alex have at the end?

Solution

there are 2 booths:

Red booth: give 2 red, get 1 silver, 1 blue
Blue booth: give 3 blue, get 1 silver, 1 red.

Suppose Alex goes to the red booth first. He starts with 75R and 75B and at the end of the red booth, he will have 1R and 112B and 37S.

Now Alex goes to the blue booth, starting with 1R, 112B and 37S. He will end up with: 1R, 2B and 103S.
  
  2017-03-26 13:11:09

Question 18

The number has the property that its units digit is the sum of its other digits, that is . How many integers less than but greater than share this property?

Solution

  
  2020-07-09 06:35:46

Question 19

The real numbers form an arithmetic sequence with . The quadratic has exactly one root. What is this root?

Solution

let r be the root. Then from Vieta's rule:
r+r = -b/a
r*r = c/a

Since c,b,a are arithmetic sequence,

c-b = b-a
c/a -b/a = b/a -1

r+r+r*r = b/a-1 = -(r+r) -1
r^2 + 4r +1 = 0

r = -2 + sqrt(3), r = -2 - sqrt(3)

We see 0
  
  2017-01-07 21:02:31

Question 20

The number is expressed in the form where and are positive integers and is as small as possible. What is ?

Solution

  
  2020-07-09 06:35:46

Question 21

Two non-decreasing sequences of nonnegative integers have different first terms. Each sequence has the property that each term beginning with the third is the sum of the previous two terms, and the seventh term of each sequence is . What is the smallest possible value of ?

Solution

From the problem, we got the following constraints:

X1, X2, X1+X2, X1+2*X2, 2*X1+3*X2,3*X1+5*X2,5*X1+8*X2

1. 5 * X1 + 8 * X2 = 5 * Y1 + 8 * Y2

We can see that X1=0 will make #1 smaller than when X1 is any other non-negative integer.
2. 8 * X2 = 5 * Y1 + 8 * Y2

Now the right side must be multiple of 8. To make this possible, the smallest Y1 = 8.
3. 8 * X2 = 40 + 8 * Y2

The smallest Y2 to make #3 smallest is 8 (given that y2 must >= y1), so smallest N is 40+64 = 104.

"We can see that X1=0 will make #1 smaller than when X1 is any other non-negative integer". Why? because if not,
5 * (X1-Y1) = 8 * (Y2-X2)
So X1-Y1 must be multiple of 8.
So X1 must be at least 9, and X2 must be at least 9 too. In such case, 5*X1+8*X2 must be larger than 45+72=117, which is larger than 104.
  
  2017-03-12 16:01:11

Question 22

The regular octagon has its center at . Each of the vertices and the center are to be associated with one of the digits through , with each digit used once, in such a way that the sums of the numbers on the lines , , , and are all equal. In how many ways can this be done?

Solution

First of all, note that J must be 1, 5, or 9 to preserve symmetry. You can try for yourself the scenarios where J is not 1, 5, or 9, and you will find it is impossible to 4 lines equal: A+E = B+F = C+G = D+H.

Assume that J = 1. Thus the pairs of vertices must be 9 and 2, 8 and 3, 7 and 4, and 6 and 5. There are 4! = 24 ways to assign these to the vertices (selecting 4 from 4, order matters, ie, permutation). Furthermore, there are 2^4 = 16 ways to switch them (i.e. do 2 9 instead of 9 2).

Thus, there are 16*24 = 384 ways for each possible J value. There are 3 possible J values that still preserve symmetry, so answer is 384*3 = 1152
  
  2017-03-12 16:25:03

Question 23

In triangle , , , and . Distinct points , , and lie on segments , , and , respectively, such that , , and . The length of segment can be written as , where and are relatively prime positive integers. What is ?


Solution

  
  2020-07-09 06:35:46

Question 24

A positive integer is nice if there is a positive integer with exactly four positive divisors (including and ) such that the sum of the four divisors is equal to . How many numbers in the set are nice?


Solution

  
  2020-07-09 06:35:46

Question 25

Bernardo chooses a three-digit positive integer and writes both its base-5 and base-6 representations on a blackboard. Later LeRoy sees the two numbers Bernardo has written. Treating the two numbers as base-10 integers, he adds them to obtain an integer . For example, if , Bernardo writes the numbers and , and LeRoy obtains the sum . For how many choices of are the two rightmost digits of , in order, the same as those of ?

Solution

  
  2020-07-09 06:35:46

Answer Keys


Question 1: C
Question 2: A
Question 3: C
Question 4: D
Question 5: B
Question 6: C
Question 7: B
Question 8: B
Question 9: D
Question 10: C
Question 11: B
Question 12: B
Question 13: E
Question 14: E
Question 15: B
Question 16: B
Question 17: E
Question 18: D
Question 19: D
Question 20: B
Question 21: C
Question 22: C
Question 23: B
Question 24: A
Question 25: E