Rob Eastaway is a recreational mathematician, author, speaker, and maths event organiser. His latest book, *Maths on the Back on an Envelope* (Amazon UK, Waterstones, Maths Gear (signed)), is all about how to (roughly) calculate pretty much anything.

## Style

*Maths on the Back on an Envelope* is an easy-going and enjoyable read. It takes the reader through some methods of approximation of numbers, while showing the reader why being able to perform such approximations is a useful skill. As well as suggesting the use of approximation to check that the correctness of answers, or to work out the approximate size of answer that should be expected, the book also makes the very good point that often an approximation is the best that we can do, and giving answers to a greater-than-appropriate level of accuracy can be misleading.

## Control

The book explains the methods of approximation in a very understandable manner, and also justifies their use and appropriateness. Throughout, the user is challenged to try out their newly found calculation skills; the solutions to these challenges are given the back of the book, alongside some discussion of some ways people go about answering them.

## Damage

The discussion of estimation and accuracy in this book is very strong, and is not something I’ve read much about elsewhere. The book sets itself apart from other books by encouraging you to actively engage in tasks related to the material being discussed, and you may find yourself finishing it with a far greater understanding of what everyday numbers mean.

## Aggression

Although many of the methods of mental or on-paper calculation discussed in this book will be familiar to the keen mathematician, this book is still an enjoyable read, and even the most capable calculator can benefit from thinking about the issues surrounding accuracy in this book. I’d most strongly recommend this book to those looking to improve their skills with numbers, or to improve their understanding of numbers around them an in the news, but there’s still plenty to enjoy here for others.