If ab is odd, then a and b are both odd. I'm not quite sure how to go about proving this. I think I need to assume ab is odd or maybe do a proof by contradicition?
Assume a and b are not both odd, then you have 3 cases:
(1) a is even but b is odd
(2) a is odd but b is even
(3) a and b are both even
in all cases, we have ab even, so the proof by contrapositive follows