(a) 4, 8 and 12

Factors of 4 = 1, 2 and 4

Factors of 8 = 1, 2, 4 and 8

Factors of 12 = 1, 2, 3, 4, 6 and 12

Comman factors = 1, 2, and 4

(b) 5, 15 and 25

Factors of 5 = 1, 5

Factors of 15 = 1, 3, 5 and 15

Factors of 25 = 1, 5 and 25

Common factors = 1 and 5.