Numbers up to 20
Primary 1 · Mathematics
Numbers up to 20
Lesson 6 of 7
13 min
Let's begin
Mutesi carries 13 small mandazi to sell at the school gate.
Five children buy one each. Her tray looks emptier now.
Mutesi wants to know how many are left, without counting them all again.
Today you will take away up to 20.
The - sign says: take some away.
Worked example
17 - 3.
17 is 1 ten and 7 ones.
Step 1 Keep the bundle tied. Do not touch it.
Step 2 Take 3 from the loose ones: 7 − 3 = 4 ones.
Step 3 Put them back together: 1 ten and 4 ones is 14.
Try 14 - 4 the same way.
Ones: 4 − 4 = 0. Only the bundle is left. The answer is 10.
Now Mutesi's tray: 13 - 5.
There are only 3 loose ones. You need to take 5.
So go down to ten first. Split 5 into 3 and 2.
Step 1 13 − 3 = 10. You have taken 3.
Step 2 2 more to take. 10 − 2 = 8.
13 − 5 = 8
Mutesi has 8 mandazi left.
Check it with adding: 8 + 5 = 13. It fits.
Careful! Some children count back like this: 13, 12, 11, 10, 9. They answer 9.
They counted 13 as their first jump. Do not count 13. Jump 12, 11, 10, 9, 8.
Careful! Others go 13 − 3 = 10. Then they take 5 away again. They answer 5.
You already used 3 of the 5. Only 2 are left to take.
Say the leftover out loud every time: 2 more to take.
One more. 15 - 6. Split 6 into 5 and 1.
15 − 5 = 10. 1 more to take. 10 - 1 = 9.
Tip
Drop to ten, then take the rest
Ten is a safe resting place. Land on it first.
Then say how many you still have to take.
Your answer must be smaller than the number you started with.
Watch: taking away, back through ten.
Do each one, then tap Show answer to check.
19 - 6 = ?
Answer: 13. Ones: 9 − 6 = 3. The ten stays.
16 - 4 = ?
Answer: 12. Ones: 6 − 4 = 2. The ten stays.
12 - 5 = ?
Answer: 7. Split 5 into 2 and 3. 12 − 2 = 10, then 10 − 3 = 7.
15 - 7 = ?
Answer: 8. Split 7 into 5 and 2. 15 − 5 = 10, then 10 − 2 = 8.
Gatete has 18 maize cobs. He sells 6. How many cobs are left?
Answer: 12. Ones: 8 − 6 = 2, so 1 ten and 2 ones.
What you learned today
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