Eight (8).
-- it's the only one that's not a multiple of 3
-- it's the only one that doesn't have two odd factors
They are 11, 13, 15, 17, 19, 21, 23, 25, 27 and 29.
30 is the even number.
I'd say "8". All others have the common denominator "3" in common.
15
The next number in the sequence 3-15-6-30-21 is -207.
21 23 25 27 29
21, 23, 25, 27, 29
17 19 21 23 25 27 29
15 is all of those things.
9, 15, 21, 25, 27
The odd composite numbers lower than 30 are:9 15 21 25 27
14 and 21, and 21 and 28 are the number pairs between 15 and 30 that have 7 as their greatest common factor.