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Multi-step subtraction word problem

Sal solves a multi-step subtraction word problem using a tape diagram. Created by Sal Khan.

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Video transcript

- [Instructor] We're told that a train traveling through Japan has 90 passengers. 52 passengers get off in Tokyo. In Kobe, another 29 passengers get off the train. No new passengers get on the train. And then they ask us how many passengers are still on the train? Pause this video and see if you can work through that before we work on this together. All right, now let's do this together. So first, what we can think about is how many total passengers there are, and then to figure out how many are left, well, you have to start with your total, take away 52 that get off in Tokyo, then take away another 29 that get off in Kobe, and then however many you have left, that's how many are still on the train. And we can imagine that with a tape diagram. So let's say that the length of this tape or the length of this rectangle represents the initial, the starting 90 passengers. So that's 90 passengers. And then how many get off in Tokyo, 52, and I'm doing this so that we can visualize it, so we can think about what it looks like. So we'll take away 52, which might be something like this. So 52 off in Tokyo, Tokyo, and then we have another 29 passengers that get off the train in Kobe. So then another 29, maybe that's something like this, get off in Kobe, so 29 off in Kobe. And so how many passengers are still left? Well, if you start with 90, 52 get off in Tokyo, 29 get off in Kobe, then this last part of the length of the tape, that would be how many are left. So right over there, this is the, how many, how many, how many left, would be the length of this red part of the tape. 'Cause we've taken off the 52 and the 29 off of the 90. So to figure that out, we could first try to figure out what is 90 minus 52? If you haven't figured that out, you could pause the video and try to figure that out. All right, well, one way to think about it, 90 minus 52 is the same thing as 90 minus 50 minus two. So I could write it this way, minus 50 minus two. And you might recognize that 90 minus 50, that's 9 tens minus 5 tens, that's going to be 4 tens. This is going to be 40. So this is going to be 40 minus 2, and then 2 less than 40, well, that's going to be 38. So this is going to be equal to 38. So if you only had that one stop after that first stop, you have 38 passengers left on the train. And so that 38 is this amount right over here. But then we know another 29 get off in Kobe. So we need to start with that 38, and 29 are going to get off, so minus 29. And we can view this as, maybe we could rewrite this, this is equal to 30 plus 8, that's 38, and we're going to subtract 20 and 9. So subtract 20 and subtract 9. This wouldn't be subtract 20 and then add 9. That wouldn't be 29. We're subtracting 2 tens and 9 ones. So we're subtracting 20 and 9. So first, we could view this as 30 minus 20, which is going to be 10. And then we're going to add 8 and then subtract 9, subtract 9. Now, what is 10 plus 8? 10 plus 8 we know is 18. Now, if we subtract 9 from 18, you might already know that that's going to be 9. Another way to think about it is if you subtract 8 from 18, you're going to be just left with 10, and then you have one more to take away. So you're going to be left with 9. So we are going to have 9 passengers left on the train. Start with 90, 52 get off, there's 38 left. Then another 29 get off. Now we have 9 left.