This is a simply stated geometry puzzle. What is the angle p in this isosceles triangle?
Here's some information about the origin of the puzzle. Following any links therein may spoil the fun if you were planning to solve the puzzle yourself.
The puzzle was originally published in the Mathematical Gazette's puzzle/problem section in 1922; it was problem number 644, published in volume 11. It is due to Edward Mann Langley, a mathematics teacher and author, and has come to be known as Langley's Adventitious Angles. It's related to multiple intersections of diagonals in a regular 18-gon; see e.g. this puzzle here on Puzzling; the "adventitious" in the name is because it seems somehow arbitrary and random that the relevant three diagonals should meet at a point, or that all the angles in the quadrangle BCDE (including the ones formed by the diagonals) should be rational numbers of degrees.