Mathematics Excercise - Math, Class 9 Video Lecture - Class 9 | Best Video for Class 9

- [Voiceover] If the trip
home takes 39 minutes
at what time does Chris have to leave
in order to be home by
a quarter after six?
A quarter after six.
So let's draw a timeline
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 six and after six.
So I'll start at five
and I'll go up to seven
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.
It's 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,
7 o'clock.
They tell us he has to be home
by a quarter after six, so
what is a quarter after six?
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 six,
and if we wanted to specify
what time a quarter after six is,
a quarter of the way
between six and seven.
We know there are 60 minues 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 had
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?
What is 39 minutes before,
before 6:15?
Now, the way my brain processes it,
I like to think about,
well, how much time,
how much time does it
take to get to the hour,
and then, how much time do you have left
to kind of go before the hour.
This part right over here,
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.
We've already gone 15 minutes back
if we go to 6 o'clock.
Then how many minutes do
we have left to go back?
Well we went 15 of the 39,
39 minus 15
is 24.
We have to go another 24
minutes before the hour.
This time right over here.
This time right over here is 24 minutes,
24 minutes before,
before 6 o'clock.
Before 6 o'clock and once again,
the way we figured it out.
Look, going from 6:15,
if you go 15 minutes back,
you get to 6 o'clock.
We had to go 39, so we have 24 left.
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?
Once again, we know that
there's 60 minutes in an hour.
What's 60 minus 24?
Let me do it right over here.
So 60 minus 24
is equal to 36.
You could either do that in your head
or you could say, "Hey look,
I could maybe borrow here."
"This is a ten. This is a five or regroup.
"Ten minus four is six.
Five minus two is three."
The way my brain does it is I say,
"Oh well, 60 minus 30 would get me to 30,
"but then I have to subtract,
"or 60 minus 20 would get me to 40.
"and then I need to subtract
"another four 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.
This time right over here
is 5:36.
When they say what time does Chris have
to leave in order to be
home by a quarter after six?
5:36.