Plot plot - Important thing to consider is that these type of questions are not asking you to know some magic formula. In fact, these questions are trying to weed-out people who memorise formula but don't understand what's going on.
They are really asking: #1 do you understand the situation, and #2 do you know enough to apply basic info to solve the problem.
If you get lucky, maybe you happen to know an appropriate formula, but if you're not lucky on the day, the question doesn't match the formula. For all these type of questions, if you actually understand the situation, and know a little bit about the subject, you can work it out. So think about the situation and what it means, then apply what you know.