A Math - Brain Teaser?

Question

The first one to get this right will get the points.*

>=>=>=>>>>> 56 27 43 23 XY <<<<<=<=<=<

What two digit number belongs where XY is?

*Please fully explain WHY... to win the points.

You WIN Ronin. (As soon as they let me pick a winner.) With people like you out there...it's obvious now that I need to drastically increase the difficulty level in my next math brain teaser. I will just do that. take a crack at the next one. Good luck.

Answer 1

12 or 21, not sure which, I'll shoot for 12 since every other number seems to start with 2.

add each number to get decreasing prime numbers
5+6 =11
2+7=9
4+3=7
etc...