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Q&A #4348

Teachers' Lounge Discussion: Decomposing numbers

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From: Chris <seanchris29@hotmail.com>
To: Teacher2Teacher Public Discussion
Date: 2004120419:49:05
Subject: Re: Decomposing Numbers

Although I am not a teacher, nor do I necessarily understand the
terminology used in many of the learning standards, I can relate to
the basic problem related to teaching subtraction with regrouping. My
daughter is studying this in school right now, and I have to admit, I
am at a loss of how you could ever go from having to rewrite the
crossed out numbers in an advanced class. 

I was educated in Germany, but unfortunately don't remember how we
were taught exactly, but the system I use for subtraction is really
"reverse addition," in my eyes. Here's an example:

   3 6
-  1 9
______

   1 7

start with the ones: count up from 9 to 16, which is 7, but because
you added up to 16 instead of 6, you add the 1 (or borrowed 10) to the
tens at the bottom.
Then do the tens: 1 (the one that was there already) +1 (borrowed)=2
count up from 2 to 3, which is 1. Voila, result is 17.

Adding is so much easier than subtracting for most kids, and you don't
have to cross out any numbers, just add a little number at the bottom
of the tens/hundreds/thousands.....

Does this make sense?

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