Numbers up to 100
Primary 1 · Mathematics
Numbers up to 100
Lesson 6 of 7
13 min
Let's begin
Byiringiro sells beans at a market in Kigali.
In the morning he fills 34 small bags. Then he fills 25 more.
He wants to know how many bags he has, and fast.
Today you will add and take away big numbers in two easy steps.
Never mix a loose bean with a whole bundle.
Worked example
34 + 25.
34 is 3 tens and 4 ones. 25 is 2 tens and 5 ones.
Step 1 Add the ones: 4 + 5 = 9 ones.
Step 2 Add the tens: 3 + 2 = 5 tens, which is 50.
Step 3 Put them together: 50 and 9 make 59.
So Byiringiro filled 59 bags.
Worked example
63 + 20.
There are no loose ones in 20. The ones stay 3.
Add the tens: 6 + 2 = 8 tens, which is 80.
80 and 3 make 83.
Only the tens digit changed. 63 became 83.
Worked example
71 + 6.
There are no bundles in 6. The tens stay 7.
Ones: 1 + 6 = 7 ones.
70 and 7 make 77.
Careful! Never add loose ones to a whole bundle.
Worked example
87 - 35.
Mutesi picked 87 coffee berries. She gave away 35.
Step 1 Take away the ones: 7 − 5 = 2 ones.
Step 2 Take away the tens: 8 − 3 = 5 tens, which is 50.
Step 3 Put them together: 50 and 2 make 52.
Mutesi has 52 berries left.
Worked example
59 - 30.
The ones stay 9.
Tens: 5 − 3 = 2 tens, which is 20.
20 and 9 make 29.
Careful! You took some away. Your answer must be smaller than 59.
Tip
Ones with ones. Tens with tens
Start on the right, with the loose ones. Then move left to the bundles.
After a take away, look again. Is your answer smaller? Good.
Add the ones, then add the tens.
Now take away: ones first, then the tens.
Work each one in two steps: ones first, then tens.
42 + 36
Answer: 78. Ones: 2 + 6 = 8. Tens: 4 + 3 = 7 tens.
55 + 40
Answer: 95. The ones stay 5. Tens: 5 + 4 = 9 tens.
96 - 53
Answer: 43. Ones: 6 − 3 = 3. Tens: 9 − 5 = 4 tens.
78 - 60
Answer: 18. The ones stay 8. Tens: 7 − 6 = 1 ten.
Keza picked 61 avocados. Then she picked 27 more.
Answer: 88 avocados. Ones: 1 + 7 = 8. Tens: 6 + 2 = 8 tens.
What you learned today
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