2013 AMC 10A Problem 13

Attempt Problem 13 of the 2013 AMC 10A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AMC 10A solutions, or check the answer key.

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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

Answer: B
Concepts:digitscasework
Difficulty rating: 1370
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 88 possible values 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.

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