Answer:
12 and 15 or
12 and 18 or
15 or 18
Explanation:
The two numbers have to both be divisible by 3, since their greatest common factor is 3. This only leaves..
12, 15, or 18 (no other numbers between 10 and 20 are divisible by 3)
The factors of 12 are: 2, 3, 4, 6, and 12
The factors of 15 are: 3, 5, and 15
The factors of 18 are: 2, 3, 6, 9, and 18
All three numbers have 3 as a greatest common factor, so we have 3 pairs of numbers they could be...
12 and 15 or
12 and 18 or
15 or 18