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Length word problem example (US Customary)

Sal shows how to solve a length word problem example. Created by Sal Khan.

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Video transcript

- [Instructor] We're told that Pilar has 85 inches of ribbon. She gives her friend, Nico, 19 inches of ribbon. How much ribbon does Pilar have left? Pause this video and see if you can figure that out. All right, now let's do it together. So Pilar is starting with 85 inches of ribbon. And I could represent that with what's sometimes known as a tape diagram. I guess you could call it a ribbon diagram in this case. So let's say the width of this rectangle or this tape or this ribbon represents the length of Pilar's ribbon. So this is 85 inches of ribbon right over here. She gives her friend, Nico, 19 inches of the ribbon. I'll do that in red. So gives away 19 inches. So I could think about giving away 19 inches. And this is just, I'm not doing it exactly, but you get a sense of things. So 19 inches might be about that much. So that's 19 inches that Pilar gives away. So how much ribbon does Pilar have left? Well, it would be this section right over here. Now how do we figure out how much she has left, this question mark right over here? Well, there's a couple of ways to think about it. You could say 19 plus the question mark is equal to 85. Let me write that down. 19 plus question mark, plus what we're trying to figure out, is equal to 85. So the amount she gave away plus the amount she has left is equal to 85. Or you could say if I start with 85 and if I were to take away 19, well, that's what I have left. So you could also say that question mark, the amount that she has left, is equal to 85 minus, minus 19. So what is 85 minus 19? Well, there is a bunch of ways that we can compute it. One way to do it is I could rewrite 85 as 80 plus 5, separate essentially the tens place from the ones place. We have 8 tens, which is the same thing as 80 and then 5 ones. And then rewrite 19 as 10 and 9. So if I'm subtracting 19, I'm really subtracting 10 and subtracting 9. Now why is that useful? Well, it's pretty straightforward to say that all right, 80 minus 10 is going to be equal to 70. Eight 10s minus one ten is going to be equal to seven 10s. And so I could rewrite this as 70 plus five minus nine. Now, if I just try to figure out what five minus nine is, we get into a little bit of trouble, because nine is bigger than five, but what if we were to rewrite nine in two parts? If we were to say look, nine is the same thing as four plus five, so if I'm subtracting nine, that's the same thing as subtracting five and then subtracting four. And this makes it a little bit easier, because we know that five minus five is zero, and so we can rewrite this as 70 minus 4. Now what's going to be? Well we know that 10 minus four is equal to six, so 70 minus four is going to be equal to 66. And we're all done. That's how much ribbon, she has 66 inches of ribbon left.