How many people in the world are picking their nose right now?

Weinstein and Adam solve this and 79 other world problems on chemistry, physics, biology and history – and all on the back of a napkin.

The book provides an easy step-by-step guide to problem-solving using rough-and-ready maths. And the authors have kindly left additional questions at the end to get readers started on their own problem-solving expedition.

(The answer, by the way, is 10 million people.)