If a, b not div. by 5, then ab not div. by 5 for a = 5k + r

[vikramtiwari]

Show that if integers a and b both are not divisible by 5, then the product ab is also not divisible by 5....where a=5k+r such as (1 <= r <= 4)

 

[royhaas]

This follows by factoring the two numbers into products of prime powers.