When we find the factos of tow or more numbers, then some factors may be found in both the numbers, these are called common factors.

(a) 20 and 28

Factors of 20 = 1, 2, 4, 5, 10 and 20

Factors of 28 = 1, 2, 4, 7, 14 and 28

Common factors = 1, 2, 4

(b) 15 and 25

Factors of 15 = 1, 3, 5, 15

Factors of 25 = 1, 5, 25

Common factors = 1 and 5

(c) 35 and 50

Factors of 35 = 1, 5, 7, 35

Factors of 50 = 1, 2, 5, 10, 25, 50

Common factors = 1 and 5

(d) 56 and 120

Factors of 56 = 1, 2, 4, 7, 8, 14, 28 and 56

Factors of 120 = 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120

Common factors = 1, 2, 4 and 8