What is the ones digit of this sum 1! +2! + 3! + ......... + 99999!?

Answer 1

1!=1
2!=2
3!=6
4!=24
5!=120
>5!=some number ending in zero, since everything would be a multiple of 5!, and 5! is a multiple of 10.
Thus your answer is 1+2+6+4 mod 10 or being simplified, 3.

Answer 2

1! = 1
2! = 1*2 = 2
3! = 1*2*3 = 6
4! = 1*2*3*4 = 24
5! = 1*2*3*4*5 = 120
6! = 720
7! = 5040


and every number after that will result in the factorial having a "0" at the ones digit.


Thus, taking the ones digit of the first 4 factorials (as the rest wouldn't matter), the answer is 3 (as 1+2+6+4 = 13)

Answer 3

ha! busted! do your own work!