Question

Find the sum of all two digit numbers which when divided by 5 yield unity as remaider

Answer 1

first such number is 11 and last is 96
so
96=11+(n-1)5
85=(n-1)5
17=n-1
n=18

hence
s=n/2(a+l
=18/2(11+96)
=9*107
=963