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Subtracting Left to Right: Alternative Method to Subtract 3-Digit Numbers

By Edited Nov 21, 2015 4 7

Never Borrow or Carry the 1 Again

In grade school we learn to subtract multiple digit numbers going from right to left.  We start by finding the difference at the ones place, then the tens place, then the hundreds, and so on.  Going in this direction is easy if the digits of the top number exceed the digits of the bottom number in each place, since you never have to borrow a 1 and carry a 1 to make the difference positive.  For example, 843 - 611 is easy to compute in your head.  You just calculate 3 - 1 = 2, 4 - 1 = 3, and 8 - 6 = 2 to get the answer 232.

3-Digit Subtraction

But things get messy when some of the digits in the top number are less than the digits in the bottom number.  For most people, it's not so easy to mentally calculate 812 - 657 or 790 - 589 using the right-to-left method because you have to carry the 1 at least once in each problem.  

For most problems in mathematics there is more than one way to find the answer, and arithmetic is no exception.  If you sometimes struggle with the right-to-left method of subtraction, you might find it easier to go in the opposite direction, which eliminates borrowing and carrying.  This article explains the left-to-right technique, using 3-digit numbers as examples. You can extend this method to figure differences between 2-digit and longer numbers.

The Left-to-Right Method

The first step of the method is to find the positive difference between the digits in each place.  For example, if we want to compute 812 - 657, the positive differences of the hundreds, tens, and ones places are respectively 2, 4, and 5.

Next, add the appropriate number of zeros to the end of the three numbers you found above according to their places.  Place two zeros at the end of the difference in the hundreds place, one zero at the end of the difference in the tens place, and no zeros at the end of the difference in the ones place.  Continuing with the example you get 200, 40, and 5.

Now you need to note whether the top or bottom digit is larger in each place of the subtracted quantities.  If the top digit is larger, leave the difference in that place alone.  If the bottom digit  larger, make the difference in that place negative.

 In the example of 812 - 657, the hundreds place difference is "normal" since the 8 is larger than the 6, so we keep the difference 200 as it is.  But in the tens place, the 1 on top is smaller than the 5 on the bottom, so our 40 becomes -40 instead.  In the ones place, the 2 on top is smaller than the 7 on the bottom, so our 5 becomes -5.

Finally take the three numbers 200, -40, and -5 and add them together.  Negative numbers are subtracted, so it's equivalent to calculating 200 - 40 - 5.  Doing this in stages gives you (200 - 40) - 5 = 160 - 5 = 155.  Thus, the difference between 812 and 657 is 155.

Another Example

Compute 507 minus 384.  First we take the positive differences of the digits in each place. 5 - 3 = 2, 8 - 0 = 8, and 7 - 4 = 3.

Next, we add zeros to these numbers according to their places.  Thus, 2, 8, and 3 become 200, 80, and 3.

Now we note which differences resulted from the top being larger than the bottom or the bottom being larger than the top.  Since the top 5 is larger than bottom 3 in the hundreds place, 200 stays the way it is.  Since the bottom 8 is larger than the top 0 in the tens place, 80 becomes -80.  And since the top 7 is larger than the bottom 4 in the ones place, 3 stays positive.

Lastly we compute 200 - 80 + 3:

(200 - 80) + 3 = 120 + 3 = 123.

Therefore 507 - 384 = 123.

Is This Easier Than the Traditional Method?

Many people find this method easier once they get the hang of it, but some people find it difficult to learn new arithmetic techniques if the old ways are ingrained.  

 

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Comments

Mar 15, 2013 7:45pm
Poster
Good idea but it also requires some additional steps and calculations. So I think school kids don't like this method.
Mar 22, 2013 1:11am
KCAllen
It does take some getting used to, but since I started practicing this method I can subtract things in my head so much faster. Everyone's brain works differently though, and some people are more proficient with the carry/borrow method.
Mar 22, 2013 1:11am
KCAllen
It does take some getting used to, but since I started practicing this method I can subtract things in my head so much faster. Everyone's brain works differently though, and some people are more proficient with the carry/borrow method.
Mar 22, 2013 1:12am
adragast
It is always nice to hear about alternative methods, thanks for sharing. Thumbs from me.
Mar 22, 2013 7:43am
chopsooy
I hated maths at school and reading this took me right back to the classroom :-) however I was suprised that I actually could do this! Right now it takes a little longer but I think it's a pretty cool allternative. Nice article made me read and stick around on the page!
Mar 22, 2013 6:02pm
curiosity44
I like the way that you are challenging traditional thinking about math. Your system is definintely something that children should be encouraged to try. The same method is not going to work for everyone. Keep up the great work!
Mar 22, 2013 10:14pm
JadeDragon
A worthy front page feature.
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