I wouldn't call this a maths question either. This is arithmetic. I learned from someone much wiser than I that "Mathematics is the art of avoiding calculation

I wouldn't call this a maths question either. This is arithmetic. I learned from someone much wiser than I that "Mathematics is the art of avoiding calculation." This is a first year university math question (taken from an exam paper which I took): A positron and an electron, each of rest mass m, annihilate, creating two photons each of energy pc. Show that in the frame S in which the electron was at rest, the angle between the directions of motion of the two photos is 2 sin^-1 (mc/p)^0.5 One of the photons then scatters off a second electron, also at rest in S, and subsequently has energy qc. Show that if the photos are now moving in the same direction, then q = p/3