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jmc

number theory intermediate

Problem

Find the ones digit of
Solution
First, observe that the units digit of is the same as the ones digit of for all positive integers . Also, observe that the ones digits of are 2, 4, 8, 6, 2, 4, 8, 6, .... Since is even but not divisible by 4, it leaves a remainder of 2 when divided by 4. Therefore, the units digit of is the second digit in the repeating block 2, 4, 8, 6, which is .
Final answer
4