I had a parent send me a note today b/c she can't understand why I am having her third grader do subtraction from right to left. She said that she learned it left to right. She can't figure out why we change the 0 to a 9. I cannot for the life of me figure out how you would do subtraction with borrowing from left to right. Can anyone explain it to me? Thanks!

I've never heard of doing it left to right either. I think I'm going to try it because now I'm curious!

When you say change the 0 to a 9, do you mean the zero that has actually become a 10 due to borrowing? If so, could you explain it to her that way - in that it's really changing a 10 to a 9, not a 0 to a 9? I could be completely misunderstanding your question though!

ETA: I just checked it out. It seems like a roundabout way of doing things with additional steps, but I think it's probably only because I'm not used to it. According the video I saw, you may still need to borrow depending on the numbers at the last step. Is this part of Chicago math? It reminds me of the multiplication from that.

Her weird way does work...be sure to stress that she must reinforce your teaching. In my experience, the parents way isn't 100% full proof. Sooner or later it breaks down.

I had a parent that learned math in Trinidad. She also did it left to right. She showed me how she did it and it did make sense. I had to teach her how to do it right to left so that she could work with her son. I explained to her that he will continue to work his problems out right to left all through school and that it is important for him to practice it the way it is being modeled in class. Although once he understood it, it did not hurt for him to know multiple strategies to solve the problem.

What we have been taught at our school is the child that can add and subtract using the left to right method has a clearer "number sense" than those that go the traditional, right to left direction. We are also allowing students to demonstrate their way before we teach either method. It has worked wonders in my classroom. As long as they can discuss and write down how they got their answer and the answer is correct, it is just showing the child's higher-order-thinking skill.

I agree with what the PP said. We use the Bridges program and it provides the students with a strong number sense. It encourages students to try a variety of strategies. There are many opportunities to use a strategy then pair share/trade methods. Maybe you could have her come in and present her method to the class!

Last edited by vttraveler; 01-27-2013 at 05:51 PM..

its a realy interesting way of doing subtraction!!! I never heard of it but after reading the posts here I checked it out... I think ut is much more difficult and tim consuming... I would encourage the student to do it the regular way!

Everyday Math has a method called Trade First. You do all of the borrowing first so everything can be subtracted second. I have several kids who go from left to right, it works, so I let them.

At first, I felt it would be too confusing to present different methods of subtraction to students, and that its better to just stay consistent. But after watching the video on youtube, I see that it's not that confusing to teach. We actually touch on that method in Saxon in 3rd and 4th grades, but don't insist the students use it. It seems to be more of an exposure to the process.
I'm going to reintroduce it to my fifth graders and see what they think!