Whole Number & Decimal - Lesson 5 - Subtraction

The ‘Why’: Why does subtraction work the way it does?

This lesson starts with a worded problem about people’s heights. This is to introduce the idea of a real world application of subtraction as well as unit conversion and problem solving skills. Next is an introduction to inverse operations. This explained as being one of the most beautiful and simplistic ideas in all of mathematics; that anything that can be done one way can also be done in reverse. This is shown through bar models and fact families.

As an extension of the work on addition, mental subtraction techniques such as partitioning and compensating are covered as an ‘I do, you do’ followed by some practice questions of each. Then, move on to written subtraction. This is one of very few slides where the answers haven’t been provided. However, a place value grid has been provided to talk through the answers and techniques. There is then some practice (with grids) that does include the answers and some problem solving questions as extension.

There is then a slide addressing a misconception that is often built in by teachers; that you “can’t take 3 from 2”. This isn’t strictly speaking true. If you take 3 from 2, you get negative 1. There is then an example of how, if you are secure in your understanding of place value, you can subtract using these numbers.

There is then explanation and practice for subtracting decimals. Although this will have been modelled earlier, this will be the students first chance to practice. Again, the first screen is an “I do, you do” (without answers) and then practice (with answers). There is then an example of how bar models and fact families can be used to solve algebraic expressions and lastly, some problem solving tasks using algebraic skills.

Activities included:

• Worded height starter
• The beauty of inverse functions
• Partitioning and compensating mental subtraction
• Written subtraction practice
• Problem solving
• Place value subtraction
• Subtracting decimals practice
• Using bar models for algebra
• Algebra problem solving