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Column Subtraction: Introduction to Regrouping

Sometimes, you will see an equation that you can't solve just by using simple subtraction. Here's an example:

    
  
   4 
  
    
  
-
2
8

    
  
    
  
   White Box   White Box

First, we do Right to Left. We cannot do 6 - 8. So, then we go to the next column. Can we do 4 - 2? Yes. Since we can't do 6 - 8, we ask: can we do 3 - 2? Yes. So, we cross out the 4 by clicking on the 4, make it a 3 by typing a 3 in the box above the 4, and put a 1 in front of the 6 to make 16.

In the examples on this page, you will not need to click on anything or type anything: the next page is a practice exercise with help.

Our example now looks like this:

    
  
    4  
  
    
  
-
2
8

    
  
    
  
   White Box   White Box

Now we go from right to left and do the subtraction. We first see the 16, which is above the 8. 16 - 8 = 8:

    
  
    4  
  
    
  
-
2
8

    
  
    
  
   White Box   White Box

Right to Left. 3 - 2 is 1.  Our equation is solved. The answer is 18:

    
  
    4  
  
    
  
-
2
8

    
  
    
  
   White Box   White Box

The process of changing numbers in subtraction when needed is called regrouping.

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