My dd has come up with her own strategy for doing 2-digit subtraction mentally. Do you foresee any future problems with her doing mental subtraction in this manner?

Here is what she does: 56-38=18.
1. She subtracts the tens first (50-30=20).
2. She finds the difference between the numbers in the ones place (8-6=2).
3. She subtracts that number (2) from the remaining tens (20-2=18) and comes up with the correct answer.

It works! What do you think?

Hello Marla,

I agree it does work; if the child remembers to subtract the higher number from the lower number and then to subtract it from the tens. As you see your child did not subtract the ones in the order it was I (6-8=-2), so some how she intuitively switched the order of subtraction and subtracted the larger from the smaller number, which makes this concept work for these types of problems.

The only thing I would watch for understanding is when the ones place is not borrowing from the ten place …
For instance, what if it was not 56-38= but instead it was 58-36= ?
Then what would she do?
50-30=20
8-6=2
20-2=18? then the answer would be incorrect and the method does not work.

or would she do
50-30=20
6-8=-2
20- -2=22? This would work and this would mean she understands how negatives work… which would be amazing!

My point is that though it works, and I would not discourage her from using a method she understands (unless the lesson is particularly teaching another method for that lesson), I would make sure that it will work for her in all the ways. So make sure she can do it both ways, 58-36= and 56-38=, and tell you how she did it. Actually, you will learn this method later in the curriculum --Level D, it is called Terry’s Method, and she should feel comfortable with it if she is figuring this out now.


Please let me know if you have any more questions. You can email me directly at Carissa@alabacus.com .

Thank you for giving your child a RightStart in Math,

Carissa
RightStart™ Mathematics by Activities for Learning, Inc.

For program questions: 888.272.3291
To place an order: 888.RS.5.MATH (888.775.6284)
www.RightStartMath.com

Our Mission: To help children understand, apply, and enjoy mathematics

Thanks Carissa. I understand your concern about my dd's "strategy" not working when there is no borrowing involved. Here is what she does:

56-38 =
50-30 = 20
6-8 = -2
20-2 = 18

58-36 =
50-30 = 20
8-6 = 2
20 + 2 =22

It works. She comes up with some different ways of doing problems so just wanted to make sure it wouldn't create future problems. Thanks!

It looks like she is showing a good sign of understanding numbers and place value.

A good mathmatician is always looking for the patterns in math, and it looks like she found one!

Have a nice day!
Carissa