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

4 : 15

THE LOGIC;

1) 12 : 9;

divide 12 by 3, we get 4. So, the operation here is division by 3.

2) 8 : 4;

divide 8 by 2, we get 4. So, the operation here is division by 2.

3) 4 : 15:

multiply 4 by 3.75, we get 15. So, the operation here is multiplication by 3.75.

4) 20 : 25;

add 5 to 20, we get 25. So, the operation here is addition by 5.

Among these pairs, the odd one out is 4 : 15, as it involves multiplication, while the others involve either division or addition.