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.

15 : 256

TE LOGIC;

1) 19 : 323,

19x17 = 323

2) 43 : 731,

43x17 = 731

3) 21 : 357,

21x21 = 357

4) 15 : 256,

15x15 = 255 not 256

Thus, 15 : 256 is the odd one.