If a non-negative integer n ends in 5, then n can be written as 10m + 5, where m is a whole number. Since 10 is divisible by 5, n can also be written as 5(2m+1), which is divisible by 5. Therefore, if n ends in 5, then 5 divides n, or 5 | n.

1. Q. Prove that if n is any non negative integer whose decimal representation ends in 5, then 5 | n.
Solution
If n end in 5, then we can write as follows. then n=10m+5, for some whole number
m since 10 = 2*5 n can be written as n = 2*5m + 5 = 5(2m+1) = 5* something which is multiple
of 5 thus 5 divides n thus 5 | n Hence it is proved