Practice problem:

The following is correct?

456 532 1087 + 36 ------ 2132

I will present how to make sure if this calculation is correct; that is, mentally calculate only each last figure:

6 2 7 + 6 ----- 26 + 2 is 8;

8 + 7 is 15 where only the last figure 5 is used;

5 + 6 is 11 where only the last figure 1 is also used; and

The obtained numeral 1 does not match 2 of the last figure in the practice problem total 2132, so we get our conclusion "The calculation for this problem is incorrect".

In this case where we made sure using each last figure only, we can determine the correctness in a probability percentage of 90.

Practically we can make sure if calculations are correct in our payments, for example, receipts from supermarkets.

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