Can anyone help on this? thank you

In problems 4 and 5, you may use rules of elementary algebra, of course. 4. Let n be any non-negative integer. Prove that if 5 divides n then 5 must divide n Hint: proof by contraposition and by cases, as in the first class?] 5. Prove, by contradiction, that 5 is not rational. You may use the fact in 4. above in your proof