2 + 7 + 1 + 6 = 16
4 + 1 + 3 + 5 = 13
5 + 3 + 2 + 1 = 11
7 + 1 + 4 + 5 = 17
If you go through the sequence you will see that the numbers add up to be a prime number except in the case of 2716 i.e. 16 which is not a prime number. Hence 2716 is the odd one....