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### Course: Class 2 > Unit 2

Lesson 5: Addition and subtraction using number line# Adding and subtracting on number line word problems

Sal uses number lines to solve addition and subtraction word problems with numbers less than 100.

## Want to join the conversation?

- Is using a number line the best way to add and subtract? Because in some cases, it seems easier to not use a number line.(19 votes)
- Whatever is better for you. A number line is just a good illustration of what actually happens when adding or subtracting. As you go up, you won't need them at all (adding with them).(15 votes)

- 0:35. Why do I have to use a number line if I can quickly solve 30 + 4 + 3 + 2 to get 39?(17 votes)
- Its a way for a first grader to show their thinking. It doesn't make them weak. We as adults can calculate quicker. First graders need a strategy to show their thinking at that age. As they get older and strengthen their mental math, they will be able to calculate a lot faster.(4 votes)

## Video transcript

- All right, so we have Ying Ying had run a total of 30
laps for Running Class, or Running Club, I should say. So Ying Ying ran a total of 30 laps. Then, Ying Ying ran four
more laps before school. So she ran four more laps before school. Three more laps at lunch, three more laps at lunch. And two more laps at recess. Ying Ying's doing a lot of running. Two more laps at recess. So what I want to do is, I want to figure out
the total number of laps that Ying Ying ran, and I'm going to do it using a number line. So let's draw a number line here. Let's draw a number line. And Ying Ying starts at 30 laps. She had run 30 laps, and then she keeps running
more laps than that. So my number line, I could start it wherever I want, but it might be convenient
to start it at 30. And obviously I could do it at 29, and then I could go 30, actually let me just
keep going up from there. 31, 32, 33, 34, 35, a few more left. 36, 37, 38, 39, and let's say 40. And I could go all the way to 41. That's how much space I have. So let's see. Ying Ying started at 30 laps. So she started at 30 laps for Running Club. Then she ran, Ying Ying ran four more laps before school. So on the number line, if I go four more, that's going to be to the right. I'm going to be increasing by four. So I'm going to be increasing
by one, two, three, four. So those four laps before school take us from 30 to 34. Then she runs three more laps at lunch. So then from 34, she's gonna
go one, two, three to 37. She's gonna go to 37. So that's what those
three more laps at lunch do for us. And then two more laps at recess. So she was at 37, and now we're gonna go one, two, to 39. So how many total laps does Ying Ying run? Well, Ying Ying runs a total of 39 laps. 39 laps, and we were able to figure it out using this number line. Now let's do an exercise
very similar to this that you might see on Khan Academy. So this one right over here. Let me get rid of the
stuff about Ying Ying so we don't get confused. So this says, and it's
a little bit smaller but hopefully you can see what it says. A red ant army started
with 100 marching ants. So we're gonna start with 100. And actually they tell us, well let me just read the whole thing. They started with 100 marching ants. Then the red army lost
three ants down a hole. So actually let me, so lost
three ants down a hole. Three ants were eaten by a bird. That's unfortunate. And one ant just blew away. I guess ants are fairly light creatures, so that makes sense. One ant blew away. Which number line shows how to find how many ants the red army has left? Well, the army started
with 100 marching ants. We underlined that over here, so we need to start with 100. We need to start with 100. This one starts with 10. So I can immediately rule this one out. And so we could just select that one, but let's just make sure it makes sense. So we start with 100, and then we lose three ants down a hole. So we're gonna go down. Remember, we're losing ants, so we're gonna go to the
left of the number line. We're gonna go to lower values. We're gonna go down one, two, three. So these are the three
ants we lose down a hole. So three down the hole. Then we have three that
were eaten by a bird, so you're gonna lose three more. That's those three right over there. It goes to 94 ants left. And then we have the one
ant that gets blown away. So that gets us down to 93 ants left. So the red army, or the red ant army, has 93 ants left. 93 ants left. And we are done.