Q&A #19397

Subtraction with regrouping

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From: Gail (for Teacher2Teacher Service)
Date: Mar 30, 2008 at 08:50:42
Subject: Re: Subtraction with regrouping

Hi Deanna,

If you still have some students that are having difficulty with the
traditional algorithm and you have used up all your ideas , you may want to
consider introducing them to an alternate method.

For instance, instead of thinking about the problem as a regrouping problem,
you might show them a method like this:

  534 - 198

If you were to add 2 to each of the numbers, you would have 536 – 200. This
is a much easier problem to solve, and it will give the same answer.

The reason it works is that you have just moved both numbers the same
distance on the number line, so the space between them doesn’t change.

Try it again:

406 – 372

In order to make the 372 become 400, we have to add 28 to it. If we add 28
to 406 we get 434. So now the problem is 434 – 400.

Of course, you could also subtract the same amount from both numbers.  So
406 – 372 could be 404 – 370, or 394 - 360.

The idea is to help students see that they can manipulate the numbers to
make a problem that they can solve. That flexibility in thinking will really
be an asset to them later.  I hope this gives you something to work with.  :-)

 -Gail, for the T2T service

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