Tim,
It's all pretty basic mental arithmatic. Sometimes the numbers involved aren't very pretty, but on the whole the maths itself isn't the challenge, given enough time most people would be able to answer all the questions without much of a problem. The difficulty comes in that they don't give you a whole lot of time, you have to work fast and are under quite a lot of pressure.
The questions themselves are generally all SDT questions, think there may have been one or two others in there, like basic addition or subtraction of degrees to work out compass headings and the like.