Problem:
How many integers between 1 and 2005 (inclusive) have an odd number of even digits?
Problem:
How many integers between 1 and 2005 (inclusive) have an odd number of even digits?
Solution:
The answer is 1002. If the units digit of an integer is even, then the units digit of is odd, while all the other digits of and are the same. Therefore, if has an even number of even digits, then has an odd number of them; and if has an odd number of even digits, then has an even number of them. Hence, exactly one of and has an odd number of even digits. Dividing the integers between 2 and 2005 into the 1002 pairs , 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.