Which number does not belong 2 8 13 15 16?

The answer depends on which one you would like it to be:

• 2, it is the only even prime;
• 8, it is the only perfect cube in the list;
• 13, because it is the only odd Prime number in the list;
• 15, because it is the only number in the list with different prime factors; and
• 16, it is the only number with an odd number of divisors.

Take your pick!