Subtraction Within 40
Thinking Platform
Look at all the marbles.
Think! What is the easiest way to subtract these marbles? Can you remember the different ways to subtract? Finger Subtraction
Subtract by using your fi ngers. 38 – 13 = ?
Think! Can you use your fi ngers to subtract these numbers?
Subtracting By Crossing Out
38 – 13 = ? Cross out 13 marbles from the 38 marbles. Then count the marbles left.
Subtracting By Counting Back
Subtract by counting back from the greater number. 38 – 13 = ?
Think! Is this a good way to subtract?
38, 37, 36, 35, 34, 33, 32, 31, 30, 29,
28, errr… I’ve run out of fingers… ,
27 26, 25! Phew!
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Subtracting By Counting On
Subtract by counting on from the smaller number.
38 – 13 = ?
Think! Do you think this is an easy way? Why?
Subtracting With Number Bonds
(a) without regrouping
38 – 13 = ?
Make number bonds for 38 and 13.
38
30
13
8
10
3
30 – 10 = 20
8–3=5
20 + 5 = 25
Think! Is it easy to use number bonds to find 38 – 19? Why?
(b) with regrouping
38 – 19 = ?
Make number bonds for 38 and 19.
38
20
19
18
10
20 – 10 = 10
18 – 9 = 9
10 + 9 = 19
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You cannot subtract 9 ones
from 8 ones! So, regroup
the tens and ones in 38.
9
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Subtracting Using Vertical Subtraction
(a) without regrouping
38 – 13 = ?
Tens
–
Ones 3
1
8
3
2
5
First, subtract the ones.
8 ones – 3 ones = 5 ones
Then subtract the tens.
3 tens – 1 ten = 2 tens
So, 38 – 13 = 25.
(b) with regrouping
38 – 19 = ?
Tens
2 –
3
1
1
Ones 1
8
9
9
First, regroup the tens in 38.
3 tens 8 ones = 2 tens 18 ones
Subtract the ones.
18 ones – 9 ones = 9 ones
Then subtract the tens.
2 tens – 1 ten = 1 ten
So, 38 – 19 = 19.
Note s
Teaching Tips
This chapter introduces the strategy of vertical subtraction with and without regrouping. Whilst reinforcing
previously taught strategies, it also highlights their limitations when subtracting numbers greater than 10. At this
point, the child should be confident with vertical subtraction with and without regrouping so that he can solve the
problems that follow.
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Fill in the blanks.
6. The numbers below show the marks scored by Gabriel, Lynn, Celine
and Dawn.
Marks: 27
23
29
28
Gabriel scored 4 marks more than Celine.
Dawn scored 2 marks more than Gabriel.
Lynn scored 5 marks more than the one with the lowest marks.
Write down the scores for each of them.
Gabriel _____ Celine ______ Lynn ______ Dawn _____
7. Desmond is thinking of a 2-digit number.
The digit in the tens place is 3 less than 5.
The digit in the ones place is 3 more than the digit in the tens place.
What number is Desmond thinking of?
______________
8. Arrange the numbers from the smallest to the greatest.
36
35
31
33
smallest
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greatest
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