Apparently the graphics department of a Thai textbook company got a little lazy one day and just grabbed something from a Google image search for their textbook (above). Turns out they grabbed an image of Mana Aoki, a Thai porn star. Cue the outrage!
It's actually pretty tame, the sort of things school boys and girls will joke about like an urban legend. It's not like she's counting strokes on the cover or the textbook has a combinatorics problem concerning the logistics of group sex.
Also, I am pretty sure that title:
is Thai for "Numberwang!"
(An image of Ms. Aoki. For "reference". But mostly so I can make numberwang jokes.)