What is a common factor of 18 and 30?

The prime factors of 18 are 2, 3 and 3

The prime factors of 30 are 2, 3 and 5 As 2 and 3 are common to both then they are common factors. Further, when numbers share common prime factors, any number which those factors can produce through multiplication is also a common factor of the two original numbers. In this case 2 x 3 = 6, so 2, 3 and 6 are common factors of 18 and 30.