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Find the units digit of 1! + 2! + 3! + ... + 100!.

 May 14, 2020
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1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7!= 5040
...

From this, you can see the units digit from 5! and beyond is 0. So the units digit of the sum of the factorials, is just the sum of the units digits of 1! - 4! which is 1 +2 + 6 + 4 = 13. Since we only need the units digit, the answer is 3.

 May 14, 2020

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