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Q&A #522

Teachers' Lounge Discussion: Teaching subtraction

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From: Jeanne 
To: Teacher2Teacher Public Discussion
Date: 2000110713:58:41
Subject: Using money to understand

Hello,
I haven't been able to come up with a way to draw a picture of the
base-10 blocks on this particular type of word processor.  But I
recently wrote a response to a parent homeschooling her daughter which
might give you an idea of what to do.

Translation:
     pennies ---------> units
     dimes -----------> longs
     dollar bill -----> flats
     10-dollar bill --> 1000 block

Let's look at this problem using money.

For now let's look at a similar problem using smaller numbers.

                    In terms of money, this problem becomes
     500  ------>   5 dollar bills
    -123  ------> - 1 dollar bill,  2 dimes, 3 pennies
    -----        --------------------------------------

In order to do this problem your daughter needs to "make change."

     500  ------>   4 dollar bills, 10 dimes
    -123  ------> - 1 dollar bill,   2 dimes, 3 pennies
    -----        ---------------------------------------

Then,
     500  ------>   4 dollar bills, 9 dimes, 10 pennies
    -123  ------> - 1 dollar bill,  2 dimes,  3 pennies
    -----        ---------------------------------------

Now she can do the subtraction.
     500  ------>   4 dollar bills, 9 dimes, 10 pennies
    -123  ------> - 1 dollar bill,  2 dimes,  3 pennies
    -----        ---------------------------------------
     377  <------   3 dollar bills, 7 dimes,  7 pennies

What I can't show here is the notation that goes with the original
problem, sorry.

Now let's look at your original problem.

     4000 ------>  4 10-dollar bills
   - 1296 ------>  1 10-dollar bill,   2 dollars,  9 dimes, 6 pennies
 
---------------------------------------------------------------------

To do this problem, she needs to make change...

     4000 ------>  3 10-dollar bills  10 dollars
   - 1296 ------>  1 10-dollar bill,   2 dollars,  9 dimes, 6 pennies
 
---------------------------------------------------------------------


     4000 ------>  3 10-dollar bills,  9 dollars, 10 dimes
   - 1296 ------>  1 10-dollar bill,   2 dollars,  9 dimes, 6 pennies
 
---------------------------------------------------------------------

     4000 ------>  3 10-dollar bills,  9 dollars,  9 dimes, 10 pennies
   - 1296 ------>  1 10-dollar bill,   2 dollars,  9 dimes,  6 pennies
 
---------------------------------------------------------------------
     2704 <-----   2 10-dollar bills,  7 dollars,  0 dimes,  4 pennies

Hope this helps. 
 -Jeanne, for the T2T service

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