Dr. Cotter on Subtraction Approaches

There are at least eight different ways to subtract multi-digit numbers. Even though a person has learned one method, it is good to explore other methods to enhance understanding.

If a child has an accurate and efficient method of subtraction, do not insist that the child change to a new method, rather encourage them to try a different method and see if they like it.

Subtraction can proceed from left to right like division. This is taught in RightStart Mathematics Level C.

subtract from left

Here subtraction proceeds from right to left, as in the traditional method used in the U.S. and Canada.

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subtract from right

Also, subtracting on side 2 of the AL Abacus makes it virtually impossible to subtract 2 from 6 as some children want to do in the following example:

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Comments

  1. I found it pretty confusing in Rightstart C when I taught subtraction. I liked learning the left-to-right subtraction, but would have liked a lesson on the “traditional” method as well (like there had been for the traditional method of addition). In the example given in this blog entry, 6829-2637, when the child gets to subtracting the tens, he will still have to trade and will then have to erase the 1 he had just written. Erasing so many numbers became cumbersome for my daughter. My in-laws looked at me funny when she subtracted and was continually erasing so I taught her the traditional method. I am glad she can know both ways! Thanks for posting this because I didn’t know if there was a reason the traditional way had been left out of the lessons.

    • Rachel Anderson says:

      Hi, Beth.

      When my kids worked through these lessons, they didn’t erase their numbers (the little ones). Just like you and I learned in school to cross off the number and put the other number on top. We didn’t erase those, but left the borrowing notations in place. Same thing with these little ones.

      After my children got very comfortable with subtracting, they would look at the top number, make all the trading needed for the equation and then do the subtraction. They made such quick work of it and it really showed their complete understanding of place value.

      Thank you for posting!

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