It annoys me, though, that there's a prescribed method of estimation, which is hard to remember. I always used to get the "wrong" answer because I picked easier-to-add approximations, even though my estimates were generally both faster and closer. (To use your example, I'd have estimated 35 or 36: the sum of two small numbers, like 2 and 3, is normally about 5; and this sum is a little bit less than (1 + 2)×12 = 3×12.)