# Left to right subtraction.

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I did a little Right Start B with ds last summer. I also later showed him how to mentally add 2 and 3 digit numbers by solving it from left to right even with regrouping. Since he learned this way it really strengthened his ability to do mental math and he can add things up mentally really well.

In school he will learn to do the standard algorithm for subtraction which for my other child really hurt her ability to mentally do math. I do not have the time or money to get Right Start for this skill but I want to teach him how to subtract mentally. I was using Math Mammoth but the way they teach it is not what I am looking for.

Edited by MistyMountain
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You're actually hitting two different things.  There are multiple strategies you can use for mental math.  You're correct that RS covers them, and of course you can google to find more.  RS C covers 4 digit subtraction with trades (regrouping), and yes they do it left to right.  Basically you do all the trades FIRST, mark them, then go through and subtract.  The easiest way to teach it is to use the station game, form the quantities, physically do the subtraction with each manipulative, then let them discover the algorithm.

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What is the station game? I did Google but I did not find as much as I hoped but I will try using one method I found and some mental math strategies on another site I found.

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https://www.sophia.org/tutorials/left-to-right-subtraction-algorithm

I used this method and he did really well with it. He was not happy with the right to left way we did it the other day from math mammoth even with the pictures but this he got right away and he started to anticipate looking if he will need to borrow and doing it. I feel he is more aware that he is subtracting tens and hundred this way too.

He told me his teacher taught him to add right to left but he adds left to right in his head. I think he could do this in school and it would not look too different. I found another site with mental strategies that we will try to. http://www.quickanddirtytips.com/education/math/how-to-subtract-quickly?page=1 I did a few today with him but that is something he has never done before and will need more practice.

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