Subtracting Big Numbers

Date: 02/27/98 at 08:07:12
From: Brittany Rees
Subject: Need help in math

How do you subtract 678-564 or 455646546475-67574657 ?

Date: 02/27/98 at 14:30:49
From: Doctor Ethan
Subject: Re: Need help in math

Hello Brittany,

Subtraction is pretty fun stuff.

I will show you how to do the first one and get you started on the 
second, and I think that you will be able to finish it.

       678 - 564

I think that it is easier to rewrite it like this:

       678
     - 564

Now what we are going to do is subtract in each column.

We will start in the right column.

     8 - 4 = 4 so we have

       678
     - 564
     ------
         4

Then we move to the next column.  7 - 6 = 1  to give us

       678
     - 564
     ------
        14

And finally 6 - 5 = 1 to leave us with

       678
     - 564
     ------
       114

and that is the final answer

     678 - 564 = 114

You can check this by adding.  114 + 564 = 678.


Now your second question is bigger, but the same ideas apply.

      455646546475
    - 000067574657
    ---------------

I added the zeros at the beginning so that things would line up.

Now we are going to start by doing the same thing we did last time.
Look at the right most column (called the one's column).

We want to look at 5 - 7  but this will give us a negative 2 which 
will confuse our method. So we are going to do a trick. We are going 
to take one "ten" from the ten's column (the second in from the right) 
and and give it to te ones column of the top number.

So what we have now looks like this:

               15
     45564654646
   - 000067574657
   ---------------


Do you see what I did?  The "1" in the "15" came from the tens column, 
so now the tens column is a six instead of a seven.  Now I think of 
the 15 as being all in the same column as the 7 of the bottom number.

So 15 - 7 = 8, leaving us with

               15
     45564654646
   - 000067574657
   ---------------
                8
  
Now for the next row in we are okay because 6-5 = 1  (remember, we 
can't go back and use the 7 on top because we already used one of 
those tens. So we have:

               15
     45564654646
   - 000067574657
  ---------------
               18

This pattern just continues. I recommend that you try it on a few 
smaller problems before you tackle a big one like this.

Have fun!

-Doctor Ethan,  The Math Forum
Check out our web site http://mathforum.org/dr.math/