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 is related to Module 2: Number and Algebra: addition and subtraction - teacher guide, Module 5: Number and Algebra: using place value to write numbers - teacher guide, Module 8: Number and Algebra: subtraction of whole numbers - teacher guide and Module 18: Number and Algebra: decimals and percentages - teacher guide.