Question

Find the digit at unit place of 3" if n is 9.
24​

Answer 1

Answer:

We have to evaluate unit's digit of the sum 1!+2!+3!+...+49!

We have,

1!=1

2!=2

3!=6

4!=24

5!=120

6!=6(5!)=720

7!=7×6(5!)= a multiple of 10.

Clearly, factorial of every number after 4 will be a multiple of 10. So, factorial value of every number after 4 will have 0 at its unit's place.

Unit digit of required sum 1+2+6+4+0+... is 3.