The inverse of an implication p ⇒ q is ¬p ⇒ ¬q (where ¬ means "not", denoting the logical negation).
Compare this to the converse and contrapositive of p ⇒ q.
Converse: q ⇒ p
Contrapositive: ¬q ⇒ ¬p
(in other words, the contrapositive is the inverse of the converse)
Then the inverse of the given statement is
"If a number is not doubled, then the result is not even."
