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.