The first step is to write the prime factors of each number. We have
12 = 2 x 2 x 3
30 = 2 x 3 x 5
72 = 2 x 2 x 2 x 3 x 3
Looking at the factors, one 2 and one 3 are common to all three list of factors. Thus,
Greatest common factor = 2 x 3 = 6
