How Many Socks Make A Pair by Rob Eastaway

Published by JR Books

Rob Eastaway’s book, subtitled “Surprisingly interesting everyday maths”, is a gentle introduction to a variety of maths problems, typified by the puzzle from which the book takes its title: how many socks do you need to pull from a draw to be sure to get a matching pair.  The problems are developed to explore mathematical creativity, to draw out the “Beauty, Discovery and Humour” as Koestler described how creativity manifests itself, or the “Ah, Aha! and Haha of mathematics” as Eastaway neatly paraphrases.

Topics include the riffle shuffle, the dimensions of A-sized paper, magic squares and the location of the centre of Britain (exploring the centre of a triangle).

Idea for the classroom: The Saddam Puzzle (Chapter 5)

100 prisoners are told that the next day, they will be buried up to their necks in sand, in a line so the one at the back can see the 99 in front, then the next can see 98 and so on.  Each prisoner will have either a black or a white hat placed on his head.  Starting at the back, each prisoner in turn then has to shout out the colour of their own hat.  If they get it right, they are spared.  If they don’t, they aren’t!

How can the prisoners form a plan to save as many people as possible?