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Addition and Subtraction
Objectives
To add near multiples of 10 using a calculator.
To spot patterns.
Resources
Calculator | 📄 1-100 grid with red multiples of 10
Vocabulary
near multiples
Today's Lesson
Introduction
Before you start the lesson, watch this Recap Video with your student.
Using the 📄 1-100 grid with red multiples of 10, revise how we can add a ‘nearly number’ to an ordinary 2-digit number, for example, 45 + 19 = 64.
Say: ‘Look at what it would be if we only added the multiple of 10 (20). It would be 65. Look how the 1s digit in 64 is 1 less than the 1s digit in 65, just like we’ve seen before.’
Tell your student that today they are going to do lots more ‘nearly number’ additions, using a calculator to help.
Model how we can add a ‘nearly number’ to an ordinary 2-digit number on a calculator by showing them how you type in, for example, 37 + 19 and that it equals 56.
Say: ‘Look how the 1s digit in 56 is 1 less than the 1s digit in 37.’
Give the calculator to your student to allow them to familiarise themselves with their operation, using similar calculations, for example, 17 + 9, 24 + 19, 35 + 29.
Main Activity
Introduce the investigation, telling them that you hope they will notice and describe some number patterns when they are adding near multiples of 10.
Your student should first choose a 2-digit number, for example, 64.
They add 19 using a calculator.
They record the addition, for example, 64 + 19 = 83.
They should repeat this at least 8 times, adding 19 to different numbers from all around the grid.
Ask: ‘Are any numbers more difficult to add to?’
You may need to suggest that they look at the relationship between the 1s digit in the answer and the 1s digit in their starting number, as well as the way the 10s digit changes.
Now ask them to add other ‘nearly numbers’ (29, 39, 49…) to chosen 2-digit numbers, still using a calculator.
They should record each addition.
Ask them to look for a pattern about how changing the number they add will affect the answer.
They should then test out their ideas and try to explain what happens.