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### Course: Early math review > Unit 5

Lesson 9: Word problems within 100- Adding and subtracting on number line word problems
- Adding two digit numbers on a number line
- Add and subtract on the number line word problems
- Subtraction word problem: tennis balls
- Addition word problem: horses
- Addition word problems within 100
- Subtraction word problem: snow
- Subtraction word problem: crayons
- Subtraction word problems within 100

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# Adding two digit numbers on a number line

Sal uses a number line to add two digit numbers. Created by Sal Khan.

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## Video transcript

- [Instructor] We're told
that Cara had a tower with 42 blocks. She added 12 red blocks, 14 more blue blocks, and 16 purple blocks. So what we want to do in this video is think about how many
total blocks Cara now has. And to help us with that, we are going to set up a little
bit of a number line here. And this number line, it keeps
going in both directions, and I'm going to set it up so this big, thick hash mark here is 40. I wanna make sure that
I can include 42 here. And you can see, we go 41, 42, 43, 44, 45, 46, 47, 48, 49, 50. So each of the thick ones are 10 more. So that's 50, 60, 70, and 80. What I'd like you to
do is pause this video, think about where 42
is on this number line, and then think about how
do we add these blocks on this number line to think about how many total blocks Cara now has. All right, now let's do this together. So let's start with her starting point, the 42 blocks right over here. So we see this is 40. So we go 41, 42. So this right over here
is Cara's starting point. That is 42 blocks. Now first she adds 12 red blocks. So you could just count
on the number line. So if you add one, two,
three, four, five, six, seven, eight, nine, 10, 11, 12, you get right over there. So this right over here is
all of us adding 12 blocks. Now where does that get us to? Well we can see that we are now one, two, three, four past 50. So we are now at 54. Right over there. And you could have done it another way. You could have just added
right from the get-go. You could have said, "All
right, I'm going to add 12, "so it takes me eight to get to 50 "and then I have four
more left, gets me to 54." And then we need to add
those 14 more blue blocks. So let's add 14 from here. One, two, three, four,
five, six, seven, eight, nine, 10, 11, 12, 13, 14, gets us right over there. So we add another 14, we get over there. And now what number are we at? Well, there's two ways to think about it. You can see that we are
one, two, three, four, five, six, seven, eight above 60, which would tell us we're at 68, or you could see that we are two below 70, which would also be 68. So we're now at 68 total blocks. And then we add the 16 purple blocks. So let's do that. Let's add one, two, three,
four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16 and we get right over there. We're very lucky that our number line just happened to have
the right number of marks right over here. So then we add 16 and
we can see that we are one, two, three, four past 80. So we're at 84 total blocks. Another way you could
have thought about it is it would have taken us
two blocks to go to 70, then we would have 14 left. Another 10 to get to 80 and
then we would have four left and then you get to 84. But now Cara has a total of 84 blocks.