What’s more, this book gives suggestions for how to find answers that are “good enough” for tricky tasks like dividing by 17. It includes brand new ways to multiply and divide irrational numbers such as pi, e, the square root of 2, and the golden ratio. It has sections devoted to ancient mathematics, and the techniques we can borrow from previous and other cultures, in order to calculate more quickly. Examples, some serious, some fun, come from everyday life or from history ― like hot dog eating competitions, the Vatican’s cricket team, the molecular weight of the molecule with the world’s longest name, and the amount of people taken by Henry VIII to arguably history’s biggest party, the Field of the Cloth of Gold. In an age of timed multiple-choice questions, the swifter you can sum, or rule out wrong answers, the better you will do. If you love to play with numbers, this book will be recreational reading. And if you ever wonder whether simple arithmetic problems can crop up in everyday life, this book provides a fresh perspective.