How do you know if you need to ungroup a ten for ones when subtracting?

In this exercise we have to use the knowledge of subtraction to explain when it will be necessary to ungroup the numbers, so we have:

The ungroup strategy is used when a unit of the positive number in the subtractions is minor than the number under it.

Regrouping in arithmetic is when you create groups of ten when operating movements in the way that adding or deduction. This usually takes place when you work together double digits. However, technically, also, it takes place some occasion you have an answer that's taller 10.

For exemple:

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The ungroup strategy is used when a unit of the positive number in the subtractions is minor than the number under it.

In the image attached, you have this case, you can observe that number 4 is less than number 8, this tend to be a problem for kids, because they think

How am I going to subtract more units than the number has?

Well, we fix this problem by ungrouping the first number, which in this case is 6, and one ten unit will be place with the 4, this will transform the numer 4 into 14, this way, the kid will be able to subtract 14-8, which is 6. Then, the 6 we ungrouped, it turnes a 5, so, we subtract next 5-2, which is 3. Giving 36 as a result.