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Setting up 2-step word problems

Sal represents 2-step word problems with equations.

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Video transcript

- [Instructor] My book is 58 pages. I have already read 13 pages. I plan to read five pages each day until I finish the book. Which equation could I use to find out how many days, d, it will take to finish reading the book? So pause this video and see if you can figure that out. Okay, so let's first think about the important information they're giving us. They're giving us the number of pages that are in the book. They're giving us how much we've already read. We've already read 13 pages. And they also tell us how much we plan to read. I plan to read five pages each day until the book is finished. And then we want an equation that could use to find how many days. And d is how we're going to represent the number of days, let me do it in the same color, it'll take to finish reading the book. All right, so if we knew how many pages we have to read and we were divided by the fact that we're reading five pages per days, well, then that would give us the number of days. So how many pages do we have left to read in the book? Well, the book is 58 pages. We've already read 13, so we could subtract out 13 of those. So this expression right over here, this is how many pages we have left in the book. Let me write that down. How many pages left. And if we wanna figure out how many days it's going to take to read these, we would divide this by how many pages we read each day. And we see that we read five pages each day. So if we divide this by five, then this whole expression, let me do it in this color, this whole expression is going to tell us how many days left. How many days left. And so this is going to be equal to d 'cause d is the number of days left to finish reading the book. Now, if we look at these choices, that's exactly what we have right over here in choice C. We can make sure that these others don't make sense. 58 plus 13, no, that would be a situation where we're reading 13 pages more than the number of pages that are in the book. That doesn't make sense. And that's happening for both choice A and choice B. It would also not make sense to multiply by five because it's not going to take you five times more than the number of pages to read the book. You're going to divide by five. You're reading five pages every day. And we are done.