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Addition and subtraction with number lines

Sal figures out which number line matches up with a 2-digit addition or subtraction problem.

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Video transcript

- [Voiceover] Which number line shows 47 plus 22? Let's see, in this first number line we're starting at 47, then we add 20 to get to 67. And then we add two to get to 69. So if you add 20, and then you're adding two, you're adding 22. So this one seems right. So let's see why these choices don't seem right. So this one, we add two to get to 49 and then two more to get to 51. Well this isn't adding 22. This adding two plus two, this is adding four. To add 22, you have to add 20 and then two. And this choice right over here, we are adding 20 and then we're adding two, but we're not starting at 47, we're starting at 74. So this is 74 plus 22, so I definitely feel good about that choice right over there. Let's do a few more examples. Which number line shows 93 minus 76? So let's see, in all of these choices, we're starting at 93. Here, in this first choice we subtract seven and then six. But it's subtracting 76, you want to subtract 70. And then you want to subtract six, not just seven. So I don't like this first choice. Here we subtract 60, and then we subtract seven. But that's subtracting 67, not 76. We gotta subtract seven tens. Seven tens, and then six ones. So here we're subtracting seven tens, we're subtracting 70. And then we are subtracting six ones. So that's definitely going to be the right choice. So let's see, maybe maybe one more just for fun. Which number line shows 12 plus 87? So all of these start off at 12, and we're gonna want to add 80 and then add 7. Eight tens and seven ones. And let's see, this one adds 80 and then we add seven, so yep. Feel good about that one right over there.