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.

1 : 2

The logic;

1) 1 : 2;

If we add 1 to 1 we get 2

2) 3 : 5;

If we add 3 to 3 we get 6 not 5

3) 7 : 25;

If we add 7 to 7 we get 14 not 25

4) 5 : 13;

If we add 5 to 5 we get 10 not 13

Thus, 1 : 2 is correct.