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Addition and Subtraction
Objectives
To add 10, 11, 20 and 21.
Resources
Abacus Workbook 1 | Small toy spider
Vocabulary
ten(s) | multiple(s) of 10 | one(s) | add | subtract
Today's Lesson
Introduction
Place Spider on 45 on the 100 square. Remind your student that Spider counts in tens.
Say, ‘Show me 45 on your beaded line. Now show me the number that is 10 more.’
Move the Spider to 55.
Say, ‘Now show me the number that is ten more than that.’
Point at 65 on the grid.
Ask how much has been added altogether (20) and challenge your student to write a number sentence to match.
45 + 20 = 65
Say, ‘Show me 51. We’re going to add 20. How can we do it?’
Discuss how we can add 10, and then 10 again, i.e. we can use counting on in tens to help.
Repeat adding 20 to other 2-digit numbers less than 80 by counting on in tens.
New Learning
Place Spider on 45 on the 100 square again. Ask your student to show 45 on their bead string and then to show the number that is 10 more.
Ask, ‘Where will Spider move to on the 100 square?’ (Answer: 55)
Explain that you actually meant to add 11! What shall we do?
Agree that we can just add 1 more to our answer by sliding spider along 1 square. Spider should now sit on number 56.
Write the matching number sentence.
45 + 11 = 56
Repeat, adding 11 to other 2-digit numbers less than 90 and writing the matching number sentences
If your student is ready, use the same process to add 21 by adding 20 and then 1 more.
This video demonstrates adding 10, 11, 20 and 21 in this way.
Main Activity
Ask your student to use the following starting numbers.
14, 32, 75, 49, 26
Then ask them to add 10, add 11, add 20 and then add 21 to these numbers. They should record these in number sentences as follows:
14 + 10 =
14 + 11 =
14 + 20 =
14 + 21 =
Don’t forget to use the spider and the 100 square to help you!
Extra Challenge
Can your student work out these:
40 + 10 =
40 + 11 =
40 + 20 =
40 + 21 =
What do they need to do differently with their spider to solve these problems?
Workbook
To help your student build upon their learning from today, please ask them to complete the activities on page 42 of Abacus Workbook 1.