EXPLANATION
The greatest common of pairs is:
6 and 3:
Prime factorization of 6: 2*3
Prime factorization of 3: 3
The prime factors common to 6,3 are
=3 [Not a valid option]
30 and 36:
Prime factorization of 30: 2*3*5
Prime factorization of 36: 2*2*3*3
The prime factors common to 30,36 are
=2*3=6 [Valid option]
24 and 48:
Prime factorization of 24: 2*2*2*3
Prime factorization of 48: 2*2*2*2*3*3
The prime factors common to 24,48 are
=2*2*2*3=24 [Not a valid option]
60 and 88:
The prime factors common to 60,88 are
=2*2=4 [Not a valid option]
18 and 66:
The prime factors common to 18,66 are
=2*3=6 [Valid option]
72 and 84:
The prime factors common to 72,84 are
=2*2*3=12 [Valid option]
The right options are 2. 30 and 36 and 5. 18 and 66