Okay, because I'm feeling generous (or is it because I've got no turns to play and am bored
) here's the first part of the solution. (This is the maths part, brace yourselves.)
Let's take a set of numbers and examine the answers we get:
52 => 52 - 07 = 45
51 => 51 - 06 = 45
50 => 50 - 05 = 45
49 => 49 - 13 = 36
48 => 48 - 12 = 36
47 => 47 - 11 = 36
46 => 46 - 10 = 36
45 => 45 - 09 = 36
44 => 44 - 08 = 36
Once you realise there are a quite limited set of finite answers (namely the 9x table) the next part of the trick should become obvious...