What is the last digit of $7^{2015}$?
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5Welcome to MathSE. Please indicate what you have tried and where you are stuck. – N. F. Taussig Feb 06 '16 at 19:02
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Possible duplicate of The last digit of $2^{2006}$ – Feb 06 '16 at 19:19
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Hint:
Consider $7^1, 7^2, 7^3, \ldots$.
The last digit of this series cycles through 7, 9, 3, 1, 7, and repeats thus.
We can see that it goes through four different last digits.
Now consider 2015 modulo 4.
Alec
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