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Addition and Subtraction
Objectives
To subtract by counting up or counting back.
Resources
Abacus Workbook 3 | 📄 0-£1 Number Line | 📄 Shopping Activity Sheet | Timer | Filming device | Assignment 11
Vocabulary
subtraction | take away | count back
Today's Lesson
Assignment
This lesson works towards Assignment 11. Please access the assignment for details about what you will need to submit to your Wolsey Hall tutor.
Introduction
Before you start the lesson, watch this Recap Video with your student.
Note: use local currency if appropriate.
Explain:
- I have 87p in my purse and want to spend 33p. How much money will I have left?
Show the 📄 0-£1 number line.
Say:
- I can count back in 10s and 1s.
Model starting on 87p and counting back three 10ps to 57p. Then show how we can subtract the 3p by counting back 3 from 57 to finish with 54p.
Say:
- I have 54p left.
Record: 87 – 33 = 54.
Explain:
- Look at the digits in 33. The 10s and 1s are both smaller in this second number so we won’t have to cross over a multiple of 10 when subtracting the ones. This makes it quite straightforward to count back to work this calculation out – we start by counting back the 10s and then count back the 1s.
Say:
- Now I have 54p in my purse and I want to spend 38p.
Point out on the line that 54p and 38p are quite close together.
Say:
- We learned how to use Frog to count up along the number line!
Frog is often a handy method to choose when the numbers are closer together, especially when the 1s digit is bigger in the smaller number.
Explain:
- We still need to find the difference between these two numbers, but this time we’ll start at 38p, and count up along the number line to reach the bigger number, 54p.
Draw and annotate each hop: hop 2p to 40p, hop 10p to 50p, hop 4p to 54p.
Add the hops: 2p + 10p + 4p = 16p.
Record: 54p – 38p = 16p.
Frog is quite straightforward because we just have to remember our number bonds to hop to the next ‘friendly number’ (a 10s number or in 10s to the next multiple of 10).
Emphasise that we have two ways of subtracting. We can count back or we can count up (Frog). Write these as two headings on the board.
Ask your student to have a go at using both methods for 72p – 55p and for 57p – 25p.
Take feedback, modelling methods as necessary for each.
Ask:
- What did you notice?
- Was there a tricky moment counting back when you had to do 22p – 5p?
- How did you do this?
Discuss your student's answers to consolidate their understanding.
Note: Your student shouldn’t be learning they can only use some strategies for some calculations and some for others. Encourage them instead to begin to recognise that they have a choice; that one strategy might be more efficient, or straightforward than another, but that there is no ‘right’ or ‘wrong’ choice.
Main Activity
Set up three tables or spaces with a sheet of priced items and a labelled purse for the following 📄 Shopping Activity Sheet:
- Count back table/space
- Frog Count up table/space
- You choose table/space.
Your student should spend 10 minutes at each table or space, completing at least three calculations.
Ensure that on tables A and B, all calculations start with the purse number, that is, every calculation has the form 66p – ☐.
On Table C, your student should start with £1 and then spend until it is all gone (see instructions on the sheet).
Set a timer for 10 minutes. After 10 minutes, your student should move to a different table to do their next set of calculations (again remind them to do at least three).
Film your student taking part in each of the table activities, for Assignment 11 (only up to 5 minutes of footage needs to be submitted).
Note: The video clip should be submitted to your Wolsey Hall tutor for Assignment 11 at the end of Step 33 Lesson 5.
Workbook
To help your student consolidate their learning from today, please ask them to complete the activities on page 9 of Abacus Workbook 3.