Find the odd number pair: 23 : 29, 19 : 25, 13 : 17, 3 : 5.

19 : 25 is the odd pair.

In 23 : 29, 13 : 17 and 3 : 5 both numbers are prime.

Each of these three pairs joins two prime numbers.

19 is prime but 25 = 5 x 5 is not prime.

So the pair with a non-prime member stands apart.