Q&A #6281

Teachers' Lounge Discussion: Addition/subtraction

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From: Carl Gladish

To: Teacher2Teacher Public Discussion
Date: 2001111802:41:09
Subject: Re: Regroup or Borrow

The left-to-right method is correct, but there is a possible
complication that could make the method more difficult to learn
properly.  Basically, in the right-to-left method, you only borrow if
you "have" to.  That is, you only borrow if the digit on the top is
smaller than the digit on the right.  If you borrow first going from
left-to-right, you may create a situation where the number on top is
more than 9 larger than the number on the bottom so that after
subtracting you have, say 12.  Then you have to carry the one to the
left and increase the already large mess by scratching out the
previous top digit and increasing it by one.  Actually, the whole
problem could cascade backwards so that every number that is written
on top has to be re-written again!  For example, say the problem is

  - 121

Then after doing the left-to-right borrowing you get:

   7 18 18 18
  -   1  2  1

Now, you simply have to go from right-to-left, or the whole thing is
just stupid.  What you get, even going right-to-left is too hard to
write without using MS Paint.  Please don't recommend that students
use this method!

Carl Gladish


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