If you set where r is 0, 1, 2, 3, or 4.

Here are a couple hints:

Some values of r work and some don't.

If ab=5p, (a>0, b>0, p prime) then either:

a=5 and b=p

a=p and b=5

a=5p and b=1, or

a=1 and b=5p

Post again if you still have trouble. It's a really interesting problem.