Speed Mathematics Simplified by Edward Stoddard

Author:Edward Stoddard
Language: eng
Format: epub, pdf
Publisher: Dover Publications
Published: 1962-03-26T16:00:00+00:00

This is a strange phenomenon, but in addition to being strange it is highly useful. It means that in finding a digit sum your eye can simply skip over any 9’s. They will not change the digit sum. The digit sum of 99999999999997 will be 7.

Perhaps your mind is already ranging ahead, wondering if digits in a number that add up to 9 behave in the same way. If the digit 9 does not change the digit sum, what about 3 and 6?

Try it and see. Find the digit sum of 361. Actually work it out. Now envision the 3 and the 6 as adding to 9, and therefore to be ignored. Cast both of them out, as you would cast out a 9. Your answer, of course, is 1—the 1 you see if you ignore the 3 and the 6 (because they add to 9) in 361.

The lesson is quite true. Since 9 will not affect the digit sum, you may ignore any 9’s you see in the number—or any combination of digits that add to 9.

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