How many cats are there in the world?Whats the chance of winning the lottery twice?And just how long does it take to count to a million?Learn how to tackle tricky maths problems with nothing but the back of an envelope, a pencil and some good old-fashioned brain power.Join Rob Eastaway as he takes an entertaining look at how to figure without a calculator. Packed with amusing anecdotes, quizzes, and handy calculation tips for every situation, Maths on the Back of an Envelope is an invaluable 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.