Answer
a. If n is divisible by some prime number strictly between 1 and $\sqrt{n}$, then n is not prime.
b. If A and B are not disjoint, then A and B have at least one element in common.
Work Step by Step
A statement of the form: $\forall x \in D$, if P(x) then Q(x),
has as its contrapositive statement: $\forall x \in D$, if ~Q(x) then ~P(x).
