Problem:
Consider the statement, "If is not prime, then is prime." Which of the following values of is a counterexample to this statement?
Answer Choices:
A.
B.
C.
D.
E.
Solution:
Since a counterexample must be a value of which is not prime, must be composite, so we eliminate and . Now we subtract from the remaining answer choices, and we see that the only time is not prime is when .
The problems on this page are the property of the MAA's American Mathematics Competitions