Question

Which is a counterexample to this conjecture?

The sum of any two consecutive integers is a composite number.

A.
16 + 17 = 33

B.
10 + 11 = 21

C.
6 + 7 = 13

D.
7 + 8 = 15

Answer 1

C. 6 + 7 = 13
because 6 and 7 are consecutive integers, but 13 is not a composite number.

Answer 2

The answer is C.

33,21,and 15 are all composite numbers. Meaning that they had factors other than 1 and themselves. 13 is a prime number because 13 only factors are 1 and itself.