How many cats are there in the world? What's the chance of winning the lottery twice? And just how long does it take to count to a million?

These questions are impossible to answer exactly, but if you're versed in the art of rough-and-ready calculations you can make a good guess. Rob Eastaway's book Maths on the back of an envelope gives you a glimpse of this art, which isn't just fun but also useful. It is the tool that people in business use for quickly checking the viability of a new project. Engineers use it to check if a proposed solution is likely to work. And commentators on statistics use it to help make sense of the myriad numbers that are thrown out by politicians, expert pundits and marketers.

With its anecdotes, quizzes, and calculation tips for every situation, Maths on the back of an envelope is an introduction to the art of estimation, and a welcome reminder that sometimes our own brain is the best tool we have to deal with numbers.

You can get a taster of the book in Eastaway's Plus article How long a line can a pencil draw? or purchase it at any good book outlet.