Take Aways

Date: 03/28/2002 at 20:20:26
From: Connie
Subject: Take aways 

Dear Dr. Math,

I have a problem with take aways. I got a question out of my book. It 
says 56 - 18 = ?  I don't know how to do it. I know first you put the 
big number on the top and then you take away. But how do you take 6 
away from 5? It's very confusing. Can you please help me? 

Thank you, 
Connie

Date: 03/28/2002 at 21:03:09
From: Doctor Peterson
Subject: Re: Take aways 

Hi, Connie.

Here's a way you might like to think about it. Think of 56 as 56 
dollars. Imagine 5 ten dollar bills and 6 one dollar bills. (You might 
even want to get out some play money and try it!)

Now we want to take away 18 dollars, which means 1 ten dollar bill and 
8 one dollar bills. We can take away the ten dollar bill easily, but 
we don't have enough ones! What do we do? We make change. That is, we 
take one of the ten dollar bills to the bank and they give us 10 one 
dollar bills instead. So now, instead of 5 tens and 6 ones, we have 4 
tens (we gave one to the bank) and 16 ones (the 6 we had and 10 more). 
Now we can take away 1 ten and 8 ones easily:

      5 tens, 6 ones
    - 1 ten,  8 ones

became

      4 tens, 16 ones
    - 1 ten,   8 ones
    -----------------
      3 tens,  8 ones

That's how we subtract numbers like this. We take one away from the 
tens and add 10 to the ones before subtracting:

      5 6  -->  4 16
    - 1 8     - 1  8
    -----     ------
                3  8

Does that help?

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/