Subtraction-fest 12 ~ Checking Your Subtractions By Casting Out Nines

If you have had to complete a subtraction problem and need to check your answer to ensure that you "got it right," there are a few methods that you could use.

You could, of course, repeat what you had already done! I.e. repeat all the same steps on paper or on your calculator ... but you risk repeating an error if that is all you do. It would be sensible to check your paper calculation using a calculator, or your calculator result by working out the answer on paper, but I want to share two alternate strategies.

The first strategy (last video) was to perform the reverse addition.

The second strategy (this video) is to "cast out nines." This is a wonderful skill that is based on number theory and you can get very fast at using it! I taught our daughter how to do this with car number plates and, while quite young, she would calculate remainders as oncoming cars passed us while I was driving. I suppose that was a rate of one every second or two in heavy traffic.

In my next (and last) video, I will explain the subtraction terminology that you may encounter in text books ... and try to 'demystify' it for you.

~~~~~~~~~~~~~~~~~~~

RATIONALE FOR THIS SERIES

Some of you have expressed quite some interest in different subtraction methods/algorithms. This resulted from my posting two lengthy videos in which I explained WHY different algorithms worked.

It became apparent, however, that I needed to create a series of much shorter videos in order to DEMONSTRATE these methods (and more) without the lengthy explanations in those first two videos.