Left-to-Right (L2R) Mental Subtraction Mojo

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Here is an interactive clip which should make left-to-right subtraction easier to understand. Check it out.

   Here is the whole method in a nutshell:

1. Start at the left, with the upper row (which is called the minuend).
   
2. Before you subtract, look at the column to the right of the one you are working on (The "neighbor to the right", or the "RN".) If the lower RN is larger than the upper RN, subtract a 1 from the minuend of the column you are working on (Not  the RN!)
   
3. Now subtract the subtrahend (lower row) of the column you are working on from the upper one (remember that you subtracted a 1 from it) . Write the difference below.
   
4. Repeat the steps for the next column (the one that had just been the RN. There will be a new RN now). Remember, if at any time you reach a column where the upper number is smaller than the lower one, you can still subtract, because the upper number has an imaginary 1 in front of it.
   
5. Keep repeating until you reached the last column, at which time you will be done.

(If these "nutshell" directions seem confusing, learn from the interactive lesson above, instead.)

Make up some of your own examples and do them now. Do at least ten. Check them.

You can also try testing yourself by clicking here. It will give you random examples, and check your answers. Do them after you have made up some of your own.You learn better when you do things actively. Don't let the computer do everything for you.There is  another thing you might want to know, and that is, "What would happen if you had to carry in two or more columns in a row?" For example:

There is another interactive lesson on the next page that will clear that up for you. Make sure you understand what you have learned above, though, before you click on it.

view next interactive clip about carrying in two or more columns in a row

If you find this method a little difficult to understand, go to the lessons on subtracting with the abax. They illustrate the same thing in a simpler way.