A few years ago, one of my students popped into my office in a very excited state. "Prof. Helfand, I just had my interview with McKinsey and Company (the leading management consultant firm) and you'll never guess what they ask!" I didn't have a clue. "The first question was 'How many fax machines do you think there are in Brooklyn?' It was just like your back-of-the-envelope questions, only easier. They asked several more and I just blew them away." This scene has been repeated numerous times in subsequent years, so it seems fair for me to advertise this chapter as offering the sort of useful skill your parents think they are paying tuition for you to acquire.