I can't figure this one out. The best I can do is this:

I've noticed that the missing numbers in the sequence are 1,4,7,11,14,17.

The missing numbers seem to repeat the pattern 1,4,7 while adding ten each time. If that were the case, then 21 and 24 should be missing.

Does anyone have an answer?

2+(1)=3+(2)-5+(1)-6+(2)-8+(!)=9(1)=10+(2)=12+(1)=13+(2)=15+(1)=16+(2)=18+(1)=19+(1)=20.... Budunk, this took me 18 seconds to figure out, thanks to the numbers 2,3,and 5. Take a look at the numbers inside the parenthesis, and tell me what you see. Don't forget that there's a number repeating its self... You'll notice once you take a look.

In parenthesis I see:

1,2,1,2,2,1,2,1,2,1,2,1,1,1,1,1...

I still see no pattern.

Ohk, I think I have a pattern, but there seems to be a mistake in the "9, 10, 12, 13" part. Try adding two consecutive numbers in the sequence, and subtracting the number to the left of these numbers.Like,
(23+22) - 21 = 24
(13+15) - 12 = 16
(12+10) - 9 = 13
(3+5) - 2 = 6

This logic only fails around that 9 area. Like, (9+8)-6 should have been 11... :(

I don't get the pattern. It's too hard and unusual.

Can someone figure it out?

is the answer 27