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Addition and Subtraction
Objectives
To add a single digit to a 2-digit number by bridging multiples of 10 using knowledge of pairs to 10 and place value.
Resources
Abacus Workbook 3 | 100 bead string | Mini whiteboard and pen | 0-100 landmarked line | 0-100 beaded number line
Vocabulary
number facts | number pairs | addition facts | subtraction facts
Today's Lesson
Introduction
Show your student 28 beads on the 100 bead string. Say, ‘We’re going to add 5 beads.’
Slowly slide a group of 5 beads along to join them.
Point out that the next multiple of 10 after 28 is 30. Ask your student to watch the 2 beads going with 28 to make 30.
Ask, ‘How many more need to be added?’ Answer: 3. Point out that 30 add 3 is a straightforward addition.
Repeat with 38 + 5, then do 68 + 5 and 88 + 5.
Repeat this process to do 25 + 7 followed by 35 + 7, 75 + 7 and 85 + 7. Then show that we can use the same fact to do 37 + 5.
Show your student the 0-100 beaded number line and explain that you are going to work out 26 + 7. Ask them to draw a label after the 26th bead and before the 27th bead.
Ask, ‘How many beads do we need to make 30? How many more do we need to add after we reach 30? What is the answer? How do you know? Where is that on the line?’
Repeat with 46 + 7 and 56 + 7.
Show that we can use the same fact to do 47 + 6 and 57 + 6.
Label 25 on a 0-100 landmarked line and say that you will add 8. Ask, ‘How many more to make 30? How do you know?’
Ask what number fact will help us (5 + 5). Ask, ‘What makes you say that?’
Draw a hop from 25 to 30 and label it ‘+ 5’. How many more do I need to add? Three more.
30 + 3 is a simple addition - we do not need to count in ones. Label 33 and draw a hop from 30 to 33. So what is 25 + 8?
Repeat with 45 + 8, 68 + 5 and 58 + 5.
This video talks through adding the single-digit number by bridging 10.
Main Activity
Write 38 + 5 = at the top of the mini whiteboard. Discuss with your student how they would find the answer.
Ask your student to write three more additions using this fact on the mini whiteboard, for example, 78 + 5, 98 + 5, 28 + 5.
Point out that we could swap the units and write 35 + 8. What is the answer to this question? Write a couple more like this.
Explain that their challenge is to find all the possible ways of adding a single digit number to a 2-digit number and getting a total which ends in 3.
Challenge your student to find a way to make sure they have found all the possibilities.
Ask, ‘Can you convince me that you have all the possibilities? How?’
When they have found all the possibilities, challenge them to discover if there would be the same number of possibilities if they add a single digit number and a 2-digit number and get an answer ending in 4.
Workbook
To help your student build upon their learning from today, please ask them to complete the activities on page 10 of Abacus Workbook 3.