Sal solves what time do you leave if you are traveling 39 minutes and need to be home at 6:15. Created by Sal Khan.
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Hope this helps!(5 votes)
If the trip home takes 39 minutes, at what time does Chris have to leave in order to be home by a quarter after 6:00? So let's draw a time line and then think about what has to happen here. This is at least how I need to visualize it in order to solve this problem. And I'm going to do the hours before 6:00 and after 6:00. So I'll start at 5:00 and I'll go up to 7:00 just to make sure that we capture everything. I can draw a straighter line than that, so let me draw it a little bit straighter. Still not perfect, but it'll do the job. So let's say that this is 5 o'clock. This is 6 o'clock. And they don't specify whether it's AM or PM. And this is 7 o'clock. They tell us he has to be home by a quarter after 6:00. So what is a quarter after 6:00? Well, it's going to be a quarter of the way between 6 and 7 o'clock. So this would be half of the way between 6 and 7 o'clock. And then a quarter would be half of that. So this would be a quarter after 6:00. And if we wanted to specify what time a quarter after 6:00 is, a quarter of the way between 6:00 and 7:00, we know there are 60 minutes in an hour. So this distance is 60 minutes. A quarter of 60 is 15 minutes. So this time right over here is 6:15. And that's when Chris has to be home. Now, they tell us that the trip home takes 39 minutes. So he would have to have left 39 minutes before this in order to get home at 6:15. So let's go 39 minutes before this to figure out the time that he would have to leave in order to be home by 6:15. So what is 39 minutes before 6:15? Now, the way my brain processes it, I like to think about, well, how much time does it take to get to the hour? And then how much time do you have left to go before the hour? So this part right over here is 15 minutes. If you go back 15 minutes from 6:15 and go to 6 o'clock, that was 15 minutes. So we've already gone 15 minutes back if we go to 6 o'clock. And then how many minutes do we have left to go back? Well, we went 15 of the 39. 39 minus 15 is 24. So we have to go another 24 minutes before the hour. So this time right over here is 24 minutes before 6 o'clock. And once again, the way we figured it out, well, look. Going from 6:15, if you go 15 minutes back, you get to 6 o'clock. We have to go 39, so we have 24 left. So then we have to go 24 minutes before 6 o'clock. But 24 minutes before 6 o'clock would be how many minutes after 5 o'clock? Well, once again, we know that there are 60 minutes in an hour. So what's 60 minus 24? Let me do it right over here. So 60 minus 24 is equal to 36. And you could either do that in your head. Or you could say, hey, look, I could maybe borrow here. This is a 10. This is a 5 for a regroup. 10 minus 4 is 6. 5 minus 2 is 3. The way my brain does it is I say, oh, well, 60 minus 20 would get me to 40. And then I need to subtract another 4 more to get to 36. But either way, if we're talking about 24 minutes before 6 o'clock, that's the equivalent to 36 minutes after 5 o'clock. So this time right over here is 5:36. So when they say, what time does Chris have to leave in order to be home by a quarter after 6:00? 5:36.