Question

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.
13 + 2 = 15

C.
11 + 9 = 20

D.
5 + 8 = 13

Answer 1

To identify which example counters her conjecture, look at the parts of each example and see if they contradict what she says.

Answer 2

i did that

Answer 3

11 (prime)
+9 (composite)
=20 (composite)
Nope - not a counterexample

11 (prime)
+2 (prime)
=13 (prime)
Nope - not a counterexample

Do the same for the others. You are looking for
(prime)
+(composite)
=(prime)

or
(composite)
+(prime)
=(prime)

Answer 4

what

Answer 5

The conjecture is that a composite + prime = composite; but if you can find a case where this is not true (that is, a counterexample), then the conjecture is false.
That is why you need to look for

(prime)
+(composite)
=(prime)

or
(composite)
+(prime)
=(prime)

This would then violate the conjecture.