Problem: Find the units digit in the following sum: 1! + 2! + 3! + .... + 9!
This is a question for middle school math. Please provide an explanation and/or an answer. Thank You in advance.
1! = 1 1
2! = 2 1 * 2
3! = 6 1 * 2 * 3
4! = 24 1 * 2 * 3 * 4
5! = 120 1 * 2 * 3 * 4 * 5
6! = 720 1 * 2 * 3 * 4 * 5 * 6
7! = 5040 1 * 2 * 3 * 4 * 5 * 6 * 7
8! = 40320 1 * 2 * 3 * 4 * 5 * 6 * 7 * 8
9! = 362880 1 * 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9
Only the first four affect the units digit; add them.
1 + 2 + 6 + 24 = 33.
The units digit is 3.