Typically I do these sums not on a scrap of paper but in my head. Not because I’m some super-genius but because they are simple. And the examples in this post for the bill as well. For most of the questions of this type (and they seem to come up several times a day), simple is better. If they are too hard to do in my head I’m probably overthinking it: too detailed, too much precision, etc.<p>Another good tool for these kinds of questions is Fermi estimation.