The Odd One Out

Which number doesn't belong in the following sequence?
3, 7, 11, 15, 19, 21

Solution: The sequence consists of numbers that are incremented by 4, except for the number 21, which is the odd one out. It breaks the pattern because it should have been 23 if the pattern of adding 4 was followed. Therefore, the number 21 doesn't belong in the sequence.