RS 4 digit subtration question

[Peony]

We are on RS C lesson 90 dealing with 4 digit vertical subration equation.

4182

- 1720

 

It starts by subracting from left rather than right, which was the way I was taught. I am completely baffled by this confusing method. It makes tradings so much more complicated. You have to erase the numbers twice from trading down. Very mistake prone method IMO.

Can someone please help by explaining why starting from the left is preferred?

Thank you!

[raceNzanesmom]

:001_huh: No ideas, but I'll bump this for you.

[HejKatt]

I recall for RS B, addition, we did an exercise with adding from the left vs adding from the right just to show that it works either way, but from the left requires more erasing/trading. Then for the main exercises, the instructions were to start from the right. I wonder if it is an exercise for RS C too, and that they will later switch over to right?

 

There's also the RightStart Yahoo group, I've seen the author/her team respond to questions there:

http://tech.groups.yahoo.com/group/RightStart/

Edited November 17, 2011 by leeyeewah

[justLisa]

You can do it in more steps, but I think because you know you will have to borrow from thousands to give to hundreds you can also do it that way.

4182

-1720

Think of busting up the thousand and now you have 10 hundreds to give to the hundreds place. I don't know how it is explaining in the book, but maybe a trick like if you look over the problem and know you dno't have to borrow for tens/ones than you can start that way? I certainly wouldn't teach it that way at first though. My DS has mentnioned this though. If you understand the concept behind this long subtraction then maybe you could do it this way.

[jplain]

I don't know if this'll help, but read a few lessons forward, and then try it out for yourself with an open mind.

I was uncomfortable with it at first, but it really is a fantastic algorithm, and my daughter has no trouble using it. :)

I like it because it helps my daughter remember what she's trading (thousands, hundreds, etc.) and why.

[crazyforlatin]

That's interesting RS would recommend it this way. I find that it's easier to do mental math if done from left to right. DD combines the thousands with the hundreds mentally. The standard way from right to left is better on paper but may keep the child from thinking of place value or thinking of playing with numbers. For example, with your problem,

 

4182

-1720

 

I would give it to DD horizontally (purely to avoid having her do it from right to left), if she cannot keep that many numbers in her head. She would subtract 17-100s from 41-100s first, with an answer of 24-100's. Or first subtract 1-1000s from 4-1000s and then subtract 7-100s from 31-100s.

 

Anyway, I've taught it both ways just so that there is mental flexibility.

[Peony]

Thank you all for your help!

This is what I got from Right Start when I asked them the question:

 

Almost all kids naturally want to do math left to right. This lesson allows the child to explore what happens when you try to do math left to right. They will have to keep erasing! It gives them a very good visual of trading from a higher place value to a lower place value. In the diagrams listed on pages 183 - 185 for the problem 4182 - 1729, the student will see the arrows pointing from one of the boxes in the thousands place to 10 squares in the hundreds place and again from the tens place to the ones place. They will visualize and experience this and understand WHY you must subtract from the right to the left. Dr. Cotter, the author and founder of RightStart math, teaches the 'why' of math. She wants kids to understand why things are done the way they are done. Because the student has explored doing math from left to right and found that it can be frustrating to constantly erase numbers, they will understand that it is better to do subtraction equations from right to left - which is demonstrated in future lessons.

[2squared]

FWIW, my oldest is in Level D and she now completes all of her trading and then subtracts from the left. I've been working through problems with her, and I like this method better.