Maths Olympiad Prep

Library / /22 of 43

, 2005

Combinatorics Difficulty 5.2 AIME, harder Prove it Italy

Problem:

How many integers between 1 and 2005 (inclusive) have an odd number of even digits?

Solution

Solution:

The answer is 1002. If the units digit of an integer nn is even, then the units digit of n+1n+1 is odd, while all the other digits of nn and n+1n+1 are the same. Therefore, if nn has an even number of even digits, then n+1n+1 has an odd number of them; and if nn has an odd number of even digits, then n+1n+1 has an even number of them. Hence, exactly one of nn and n+1n+1 has an odd number of even digits. Dividing the integers between 2 and 2005 into the 1002 pairs (2,3),(4,5),(6,7),,(2004,2005)(2,3),(4,5),(6,7), \ldots,(2004,2005), we deduce that exactly 1002 of them have an odd number of even digits. Since 1 has no even digits, and therefore is not to be counted, the answer is 1002.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.