In Choose odd Number Pair / Group, certain pairs / groups of numbers are given out of which all except one are similar in some manner while one is different. The numbers in these similar pairs may have the same property or may be related to each other according to the same rule. The candidate is required to choose the odd pair / group.
(81 - 63) , (24 - 48) , (21 - 15) , (13 - 39)
Justification: This is the only group in which the sum of the digit of both the numbers is the same.