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Addition and Subtraction
Objectives
To subtract a single-digit number from a 2-digit number, bridging 10.
Resources
Abacus Workbook 1 | Blank cards or paper | 📄 100 square | 📄 0-100 beaded line
Vocabulary
subtract | bridge 10 | multiple of 10
Today's Lesson
What to Get Ready
Write the following calculations on separate blank cards / pieces of paper:
Set 1: 22 – 5, 27 – 5, 38 – 4, 33 – 4, 22 – 6, 27 – 6, 48 – 6, 53 – 7
Set 2: 23 – 5, 37 – 5, 28 – 4, 54 – 5, 82 – 6, 77 – 6, 28 – 6, 13 – 7
Introduction
Take out the 100 square grid that you coloured in in the last lesson. Remind your student that the numbers they coloured in are special numbers because they are ‘multiples of ten’. Ask your student to say all the multiples of 10 in order: 10, 20, 30...
Write 32 - 6 on a piece of paper.
Point to 32 on the grid. Say: ‘If we count back 6, we will cross a multiple of 10 (30).’
Count back two (to 30) and then 4 more (to 26) on the grid. Write the answer to complete the subtraction. 32 - 6 = 26.
Talk with your student about how they are still Targeting the 10s but ‘backwards’ as they are taking away.
Repeat using your 📄 100 square to find 45 – 7 [45 – 5 = 40, then 40 – 2 = 38].
Next, show your student the calculations that you have written on the Set 1 cards.
Show your student 2 cards from the Set 1 cards. Ask your student which calculations will cross a ‘red’ number (a multiple of 10).
Ask your student to explain how they know. See if they are right by using the 📄 0-100 beaded line.
Ask your student to sort the remaining cards into two groups:
- Group 1: will cross a multiple of 10
- Group 2: won’t cross a multiple of 10.
Main Activity
Now, give your student the Set 2 calculation cards.
Tell them they can choose to use a 📄 0-100 beaded line or their coloured in 📄 100 square to help them solve the calculation.
Ask them to answer each subtraction. They can draw ‘hops’ on their beaded line or 100 square grid to help them.
This video demonstrates using the number line to cross through a multiple of 10 when subtracting.
Workbook
To help your student to recap upon previous learning from this course, please ask them to complete the activities on page 30 of Abacus Workbook 1.