Consider the first 8 calculations of 7 to an exponent:
7^1 = 7
7^2 = 49
7^3 = 343
7^4 = 2,401
7^5 = 16,807
7^6 = 117,649
7^7 = 823,543
7^8 = 5,764,801
Take a look at the last digit of the first 8 calculations:
7, 9, 3, 1, 7, 9, 3, 1
The 7, 9, 3, 1 repeats through infinity.
So every factor of...