Question

Common factors of 20 and 42

Answer 1

2 is a common factor for both of them.

Answer 2

They are 1 and 2, because the prime factorization of 20 is 2*2*5, and the prime factorization of 42 is 2*3*7. 
You can find the prime factorization of any number at this website, for future reference: http://www.calculatorsoup.com/calculators/math/prime-factors.php

I hope this helps you.