If ak = 5 +52 + 53 + 54 + … + 5k , for what value of k will ak be divisible by 10?

Question

If ak = 5 +52 + 53 + 54 + … + 5k , for what value of k will ak be divisible by 10?

Answer 1

Since 5+2+3+4+5+6+7+8=40--->is divisible by 10,

        5+52+53+54+55+56+57+58=390--->is divisible by 10.

k=8

 

 

The way to solve these kind of problems is to keep adding numbers.