Main content

### Course: 2nd grade > Unit 6

Lesson 4: Length word problems# Length word problem example (US Customary)

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

## Want to join the conversation?

- Is the ? in the problem a variable?(1 vote)
- Yeah, this is just like basic algebra(2 votes)

## 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.