Does that statistic make sense? How much will this round of drinks come to? And what are the chances of winning the lottery? Twice? Learn how to do complex maths with nothing but the back of an envelope, a pencil and some good old-fashioned brain power. Doing mathematics without a calculator seems like an unnecessary feat in the twenty-first century. These clever little electronic devices have helped to take away the drudgery of arithmetic, and given all of us the power to perform in a few seconds what might previously have taken hours to work out. But much of the time, we don't need the exact answer with its endless decimal points - we just need to know whether something is in the right ballpark. Rob Eastaway's clever mathematical techniques do exactly that, as he takes an entertaining look at how to figure without a calculator. From spotting misleading statistics in the news and bad deals, to estimating how long you'll be standing in that slowly crawling queue, this book is packed with amusing anecdotes and handy back-of-the-envelope calculations for every situation. Most importantly, this book is a timely and welcome reminder that sometimes our own brain is the best tool we have to deal with numbers.