Ways to Teach Side by Side 2 Digit Subtraction

[notellin]

My son has not been dx with a math LD, but he clearly struggles in this area. He's in 2nd grade. I'm trying to teach him to do side by side double digit subtraction in his head (not lining them up underneath each other). For example 56-23 = _____ in his head. He's really frustrated. Has anyone seen a helpful website?

I tried having him turn it into addition. For example for 56-23 = _____, count by tens from 23 to 53 and add the tens up, and then add the ones from 54 to 56, and then combine the tens and ones. If he's going to get this method we are going to have to really work at it. Maybe someone has some ideas on a different approach to teaching this. Manipulatives maybe? I don't know.

Another idea I saw counting backwards by tens and then ones, but that seems pretty confusing so I did not try it. He's better at addition, so I thought changing it into addition made more sense.

Another idea I saw was adding the same number to both double-digit numbers to get to a multiple of ten for the lower number. For example 56 + 7 = 63 and 23 + 7 = 30. So now you are subtracting 63 from 30, which is suppose to be easier, but it doesn't seem easier to me unless you are already good at subtraction. He's on crutches when it comes to subtraction.

Does anyone have any ideas? I am not looking forward to tonight. I started going through my paperwork last night searching for the number of a tutor that I know. The poor kid was in tears. I know that I was never able to subtract side-by-side double-digits in my head until I was an adult.

[charliegirl]

Why on earth does he need to do that? I'll wait to see what advice someone else comes up with as I seem to have trouble explaining the way I do it. If no one else explains that way, I'll try my best to make it easy to understand.

Check this out. The one most pertinent is number 3, splitting up numbers. This site makes math seem much easier. www.bbc.co.uk/skillswise/numbers/wholenumbers/addsubtract/mental/factsheet.shtml

SueJ who does the education chats at Nethaven has found ways to simplify math and make it easy to understand. When they have another education chat, you may want to go there and talk to her.

[bugsmom]

Notellin...I was wondering...are they expecting the 2nd graders to know how to do this type of math problem in their heads or are you just wanting him to be able to do the mental math?

The reason why I ask is that I help the 3rd grade teacher in my sons school two days a week with math time. When the children have these types of math problems (written across instead of up and down), we have them write them out up and down, and then solve the problem. Even in third grade she doesn't expect them to be able to solve them with mental math. Not to say that some of the kids can't do it, they can, it's just that a lot of children will not use the correct numbers to solve and they get confused. I'm surprised that in 2nd grade they would expect the kids to be able to solve double digit substraction in this way. That's got to be really hard for him.

I don't really have any suggestions...sorry. I hope homework time wasn't too bad for you!

[notellin]

Yes, the school is asking that the 2nd graders be able to do this in their head. After I posted, I realized that a friend of mine writes educational math books so I asked her. This is what she said:

The Horizontal (or algebraic) format arithmetic method makes it easier to segue into algebra, since algebraic equations are written this way, as opposed to vertical format arithmetic, which is just a dead end, not leading into anything.

That said, I'm not sure why he needs to be able to do this method in his head. It is difficult. Often, we suggest that vertical format be used for doing/ solving the calculation, but horizontal format should be used to write/ record the equation. This basically means the kids just need to understand that the vertical and horizontal forms are the same, and if he can read the horizontal form, translate this in his head to vertical form to do the calculation mentally, and then write the solution horizontally, that works.

Otherwise, see about having him look at the numbers in each place separately. Using your example, 56 - 23, have him look at the ones place: 6 - 3 = 3; then have him look at the tens place: 5 - 2 = 3 (keeping in mind that since this is the tens place, the number is actually 30, not 3). He should be able to write this as 33 with no problem.

However, a problem will arise when he is given an equation that requires carrying from the tens to the ones place. For example, 56 - 29, where you need to cross out the 5 in 56, make it a 4, and "carry" that 10 over to the 6 to make it a 16. It is very difficult to even do this on paper horizontally, let alone mentally. This is why we recommend using the vertical format for calculating. The important thing is that kids need to understand that the vertical and horizontal forms are equivalent, so they can translate between doing the work and reading and recording the information.

[charliegirl]

She just explained the way I do it much better than I can. I do find that the easiest way. Now, I'm off the hook.

[crazyhouse]

notellin from my own expierence in struggling with math problems. The first method you mentioned was the easiest for me to grasp. It didn't happen overnight (weeks of blood sweat and tears)

The other thing that was very helpful was we used a lot of money to help pennies and dimes mostly. I had an easier time having some sort of real image to guide me thru the concept of ones tens and one hundreds. I hope I helped you good luck!!

[notellin]

Oh, I like the idea of using coins as manipulatives to teach double-digit subtraction, even though you are not counting back change. Thanks.