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## Elapsed time

## Video transcript

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.