Cynthia made the conjecture that the sum of any prime number and any composite number is a composite number.

Which equation is a counterexample to her conjecture?

A.

11 + 2 = 13

B.

5 + 8 = 13

C.

13 + 2 = 15

D.

11 + 9 = 20

Well,

We will need an equation in the form of:

(prime no.) + (composite no.) = (prime no.)

Options C and D immediately are rejected because the result is a composite number.

Option A looks like it could be a counterexample, but 2 is a prime number.

That leaves option B, which is the correct option.