For how many integers does the decimal representation of end with a 1?
Solution
We claim that this is only possible if has a units digit of 1. Clearly, it is true in these cases. Additionally, cannot have a units digit of 1 when has a units digit of , or 8. If has a units digit of 3 or 7, then has a units digit of 1 if and only if , a contradiction. Similarly, if has a units digit of 9, then has a units digit of 1 if and only if , also a contradiction. Since there are 202 integers between 1 and 2013, inclusive, with a units digit of 1, there are 202 such which fulfill our criterion.
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