The modules are written for teachers. They provide the mathematical background to enable teachers, together with other resources, to plan the development of topics in their classrooms. This module provides practical examples of the applications of straight-line graphs and demonstrates the usefulness of mathematics. It is related to Module 18: Number and Algebra: decimals and percentages - teacher guide, Module 17: Number and Algebra: the unitary method - teacher guide and Module 22: Number and Algebra: consumer arithmetic - teacher guide.