The second number in the given number-pairs is obtained by performing certain mathematical operation(s) on the first number. The same operation(s) are followed in all the number-pairs EXCEPT one. Find that odd number-pair.

64 : 6

The logic followed here Is :

option 1      64 : 6

                  8² = 64

                  8 - 3 = 5 but 6 is given

 option 2       25 : 2

                  5² = 25

                  5 - 3 = 2

option 3       225 : 12

                  15² = 225

                  15 - 3 = 12

option 4      169 : 10

                    13² = 169

                      13 - 3 = 10