2001 AIME I Problem 1

Below is the professionally curated solution for Problem 1 of the 2001 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2001 AIME I solutions, or check the answer key.

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Concepts:digitsdivisibilitycasework

Difficulty rating: 1950

1.

Find the sum of all positive two-digit integers that are divisible by each of their digits.

Solution:

Let the number be 10a+b10a + b with tens digit aa and units digit b.b. Since a10a+b,a \mid 10a + b, we need ab,a \mid b, so b=kab = ka for some positive integer k.k. Since b10a+b,b \mid 10a + b, we need b10a,b \mid 10a, that is ka10a,ka \mid 10a, so k10.k \mid 10. Because b=ka9,b = ka \le 9, only k=1,k = 1, 2,2, and 55 are possible.

For k=1k = 1 the numbers are 11,22,,99,11, 22, \ldots, 99, with sum 1145=495.11 \cdot 45 = 495. For k=2k = 2 they are 12,24,36,48,12, 24, 36, 48, with sum 120.120. For k=5k = 5 the only one is 15.15.

The total is 495+120+15=630.495 + 120 + 15 = 630.

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