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Column Subtraction: Regrouping Continued

Sometimes, you will see an equation that will require you to add a 1 in front of a number - and then reduce the new number by 1. Here's an example:

    
  
    
  
   9 
  
    
  
    
  
-
2
8
7

   
 
  
 
  
 
  
   White Box   White Box   White Box

Right to Left. Is 2 - 7 zero or greater than zero? No. Is 0 - 8 zero or greater than zero? However, 9 - 2 is zero or greater than zero: it equals 7.

We went from Right to Left and couldn't change things so that we could get answer. So, now, we retrace our steps. We make the 9 into an 8 and put a 1 beside the 0 so that we now have a 10:

    
  
    
  
    9  
  
    
  
    
  
-
2
8
7

   
 
  
 
  
 
  
   White Box   White Box   White Box
   White Box   White Box

We now make the 10 into a 9 and put a 1 before the 2 to get a 12. Our equation is now:

    
  
    
  
    9  
  
    10  
  
    
  
-
2
8
7

   
 
  
 
  
 
  
   White Box   White Box   White Box

Now that we know we can do the subtraction, let's subtract. Right to Left. 12 - 7 is 5:

    
  
    
  
    9  
  
    10  
  
    
  
-
2
8
7

   
 
  
 
  
 
  
   White Box   White Box   White Box

Again, Right to Left. 9 - 8 is 1:

    
  
    
  
    9  
  
    10  
  
    
  
-
2
8
7

   
 
  
 
  
 
  
   White Box   White Box   White Box

Finally, on the left, 8 - 2 is 6.  Our equation is solved. The answer is 615:

    
  
    
  
    9  
  
    10  
  
    
  
-
2
8
7

   
 
  
 
  
 
  
   White Box   White Box   White Box

So, the answer is 615.

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