Mini Mathematics
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Use parts within 20; change ones without renaming, or add and subtract tens.

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  • Primary 1 Mathematics 2021
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Pause after each idea, then explain it in your own words before moving on.

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Keep the easy parts together

For 32 + 5, keep the three tens and add the ones: 2 + 5 = 7. The answer is 37. No new ten is needed.

For 32 + 20, add tens to tens: three tens and two tens make five tens. Keep the two ones. The answer is 52.

Compare 47-3 = 44 and 47-20 = 27. Taking away ones changes the ones here. Taking away two tens changes the tens.

Make ten when it helps

For 8 + 7, split seven into two and five. Use two to complete ten: 8 + 2 = 10. Five is left, so 8 + 7 = 15.

For 15-7, you can take away five to reach ten, then take away two more. 15-5-2 = 8. You took away seven altogether.

These two calculations check each other: 8 + 7 = 15 and 15-7 = 8.

Add three small numbers

For 6 + 8 + 4, joining six and four makes ten. Then add eight: 10 + 8 = 18.

You can change the order when adding these groups because every object is still counted once. Do not change a subtraction into a different story: 12-5 is not the same as 5-12.

Check your understanding 1 Choose what to join first

Find 7 + 5 + 3. Which pair helps you make ten?

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7 + 3 = 10, then 10 + 5 = 15. Joining seven and three is a useful choice, not an extra group of ten to add on top of the original numbers.

Keep every group in the total

There can be more than three numbers to add. For 2 + 5 + 8 + 0, join two and eight to make ten. Then add five. The zero adds no objects, so the total is 15.

You can also add in the order given: 2 + 5 = 7, then 7 + 8 = 15, then 15 + 0 = 15. Both ways include every group once. Do not stop after adding just the first two numbers.

Check your understanding 2 Four groups

Find 4 + 7 + 0 + 6. Choose a pair to join first, then include the other numbers.

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  1. 4 + 6 = 10, then 10 + 7 + 0 = 17. The zero changes neither the ten nor the final total.

Check your understanding 3 Use the kind of part that changes

Find 54 + 30, 54 + 4 and 54-20. Say which place changes in each.

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84, 58 and 34. The first and third change tens. The second changes ones. None of these calculations needs an exchange.

Check your choice

Find 9 + 6 by making ten. Then check by taking nine away from your total. If you get six, the two calculations agree.

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Split six into one and five: 9 + 1 + 5 = 15. Check 15-9 = 6.

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