The Century Maths series is a flexible range of mathematics materials for pupils aged 11-16 (Years 7-11), developed by a team based at the Institute of Education, University of London. The materials have been designed and tested to fit the requirements of the National Curriculum. They provide support through Key Stages 3 and 4 and relate closely to the Non-Statutory Guidance for Mathematics. The Focus Books in the series aim to develop techniques and ideas in the four topic areas of number, algebra, shape and space, and handling data, and to continue the investigative approach of the Theme Books, alongside which they are intended to be used. This volume is one of the 12 Focus Books for Years 7 and 8.