Speedy and accurate addition (Part III)


Now, it is easy to understand how this property of the number nine can be used to check addition. If the separately calculated remainders of nine for each addend match with those for the final total, the addition is correct. See this in the two examples given here.

However, it should be remembered that the calculation may be correct, but it is not necessarily so. The check may mask an error if two digits in the sum have been accidentally reversed, for example, if the actual total is 15469 and not 15649. In this case, we will have the same remainder for both the numbers.

In the same way we have used the specialty of division by nine to check accuracy of addition, we can use the specialty of multiplication by eleven to speed up addition. Let us do it in the next article.

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