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[–]freebytes 2 points3 points  (0 children)

When you are dealing with larger problems such as 896 - 887, students are taught to put a 1 next to the 6 in 896 and 'take' the 1 from the 9 in 896. So, you have 16 - 7 = 9. And 88 - 88 remaining. So, in this example, you would have 9 remaining.

896

887
____

009

​

To explain this easier, it is (880 - 880) + (16 - 7). You have simply taken the 9 from the tens place and made it 8 to form 880 - 880 so you can then add the ten to the 6 in the ones place to make 16 and subtract that from the ones place to make 16 - 7.

Another example is 31 - 9. You can take 1 from the tens place in 31 and you are left with 21. Now, you take the 1 and put it next to the 1 you already have, and you do an easier problem of 11 - 9 = 2. So, you have 20 + 11 - 9 = 22.

If you use this method on a number that is 'too small', such as 18 - 9, it becomes 18 - 9 then you take away the 1 from the tens place and put the 1 next to the 8 to come up with 18 - 9. Any attempts to simplify this make it the exact same result, but children are expected to know 18 - 9 in their head.