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Q&A #522

Teachers' Lounge Discussion: Teaching subtraction

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From: Don <dpottruff@hotmail.com>
To: Teacher2Teacher Public Discussion
Date: 2004011415:31:03
Subject: Re: borrowing in subtraction

Before a child can really understand the rule or algorithm of
borrowing, they need to understand what the borrowing represents.  

Base 10 manipulatives and a base 10 mat are really good for this.
You can see the materials at
http://www.shnta.com/Product/Mathematics/Manipulatives/SR-0750.htm

If they work with questions with the manipulatives where they don't
have to borrow and get used to what the numbers actually mean, they
can discover the difficulty when they are given problems where they do
have to borrow.  Children can frequently come up with the idea that
they need to borrow themselves using these materials.  They should
also use the blocvks to add to get used to the concepts.

You can see the base 10 mat at
http://www.didaxinc.com/shop/productdetails.cfm/ItemNo/31-4292.htm

You don't need to have commercial materials for this.  Popsicle sticks
gathered into hundreds and tens can work.

You could also just cut out base 10 materials, there's templates at:
http://math.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fmason.gmu.edu%2F%257Emmankus%2Fwhole%2Fbase10%2Fbaseten.htm

There's also a program at
http://matti.usu.edu/nlvm/nav/frames_asid_155_g_2_t_1.html

which is very well done.  Doing it by hand is probably better because
you actually take away they blocks when you subtract, not cancel a
blue on with a red one as in the program.


The basics of all this is that the student has to visually see that
when they cross out and borrow, they're really changing a ten for 10
ones.  There's some more info at 

http://www.michigan.gov/scope/0,1607,7-155-13481_13482_13484-76410--,00.html

which should help.


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