Sorry to steal the easy one. But I can legitimately do this for all numbers up through 99 in my head. Good ol' Trachtenberg.
Damn I've been out of it so long just to try and swipe one of the easiers, cause I get do the others... at least not without paper, and if I try in my head I'll probably make an error.
Was ist Trachtenberg?
I say that in German cause it sounds very German or at least Russian... but what is it?