## #7 Modeling subtraction on a number line

Date: *December 14th, 2015*

By: *Tom Schersten*

**Tom Schersten**: This video is going to model how to do subtraction on a number line. I am starting with 42, and I am going to be subtracting 27.

I’m representing the 27 to be subtracted in yellow blocks, so that we can keep track of how many we have subtracted so far.

I’m going to start with my 42 on the right side of the number line, so that we can subtract going to the left. I’m making my dot and writing down the 42 that I’m starting with.

The first thing I would like to subtract is just 2, so I ‘m drawing my line to show I am subtracting 2. When I use these two as markers for the 2 I am subtracting, after I do that I see that what I am left with is 40.

The next thing I would like to subtract is 20, so I’m drawing the line to show that I am subtracting 20. When I take this 20 and subtract them out, I see what we are left with is just 20.

The next thing I’m going to do is subtract the 5. To do that, I’m going to take a 10 stick and trade it in for 10 1s. These yellows will mark the five that I am taking away. When those are removed, what we have left is 15.

Let me show you a different way of doing that same problem. Again, I will be starting with 42 on the right side. I’m writing down my 42 . The 27 that I need to subtract is again represented in yellow. This time, I think I would like to subtract 20 in my first step.

I’m writing down subtracting 20, and I’m using my two 10s in yellow to mark the two orange 10s we’re subtracting. After we subtract that 20, we are left with 22.

What I would like to subtract next is 2. I’m picking two of my yellows up to mark the 2 orange that we’re subtracting. After I subtract those two, I have a total of 20.

I now have to subtract five more, and I think I’m going to take the 5 out of one of my 10 sticks and just get my change. Here is the five I’m going to pay, and here’s the five I get to keep. I’m grabbing those 5, putting them with my 10 and I have a total of 15. That is the answer.

The next video is going to show how to model subtraction in a more visual spatial context.