Select the option that is related to the third term in the same way as the second term is related to the first term and the sixth term is related to the fifth term.
12 : 20 :: 30 : ? :: 56 : 72

where a and b is factor of first number

According to the logic number series will be:

20 = (3+1) (4+1); where 3,4 is the fraction of 12

? = (5+1) (6+1); where 5,6 is the fraction of 30

? = 42

72 = (7+1) (8+1); where 7,8 is the fraction of 56

Thus, the correct option is (a) 42