Maths Olympiad Prep

Library / /103 of 860

Number theory Difficulty 4.8 AIME Find the answer

For how many integers 1k20131 \leq k \leq 2013 does the decimal representation of kkk^{k} end with a 1?

A number or a short expression. Spacing and $ signs are ignored.

Solution

We claim that this is only possible if kk has a units digit of 1. Clearly, it is true in these cases. Additionally, kkk^{k} cannot have a units digit of 1 when kk has a units digit of 2,4,5,62,4,5,6, or 8. If kk has a units digit of 3 or 7, then kkk^{k} has a units digit of 1 if and only if 4k4 \mid k, a contradiction. Similarly, if kk has a units digit of 9, then kkk^{k} has a units digit of 1 if and only if 2k2 \mid k, also a contradiction. Since there are 202 integers between 1 and 2013, inclusive, with a units digit of 1, there are 202 such kk which fulfill our criterion.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.