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.