2007 AMC 10A Problem 25

Below is the professionally curated solution for Problem 25 of the 2007 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2007 AMC 10A solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:digitsmodular arithmeticbounding to limit cases

Difficulty rating: 2200

25.

For each positive integer n,n, let S(n)S(n) denote the sum of the digits of n.n. For how many values of nn is n+S(n)+S(S(n))=2007?n + S(n) + S(S(n)) = 2007?

11

22

33

44

55

Solution:

If n2007,n \le 2007, then S(n)28S(n) \le 28 and S(S(n))10,S(S(n)) \le 10, so n20072810=1969.n \ge 2007 - 28 - 10 = 1969.

Since n,n, S(n),S(n), and S(S(n))S(S(n)) all leave the same remainder modulo 99 and 20072007 is a multiple of 9,9, each must be a multiple of 3.3.

Checking the multiples of 33 between 19691969 and 2007,2007, the condition holds for 1977,1980,1983,1977, 1980, 1983, and 2001.2001.

So there are 44 values of n.n.

Thus, the correct answer is D.

Problem 25 in Other Years