Number Sequence Question

[cendar]

This question was in a test, and I can't figure out the pattern.
"What is the next number in this sequence: 19-2-3-21-5-6-15-13-?"
There is no list of answers
I'm dying to know.
thank you in advance

 

[stapel]

"What is the next number in this sequence: 19-2-3-21-5-6-15-13-?"

"The" answer is whatever the author had in mind while composing the exercise. But any pattern which you can justify mathematically, or at least logically, should be acceptable.

What are your thoughts? What have you tried so far? Is this for the competition mentioned here?

 

[cendar]

Well, it is from the competition (nothing big: t-shirts, cup are prices). It is mentioned as very hard question. In previous set of questions nobody solved one of the problems. The solution for this set will be probably in a week. I have now only this idea 19 + 2 = 21 and 21 - 6 = 15 and 15 + 13 = 28, and solution is 27, but i doubt that this is correct.