They are 11, 13, 15, 17, 19, 21, 23, 25, 27 and 29.
30 is the even number.
15
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
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.
The odd composite numbers lower than 30 are:9 15 21 25 27
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.