Hello. I'm Jessica Michaelson, Math
Support Teacher at Duckett's Lane
Elementary School.
And this is Math in Minutes.
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Today we're going to be talking about
elapsed time.
That is when a specific activity starts,
how much time has gone by,
and when that activity ends.
If we choose to use an analog clock when
doing this,
it's often challenging to keep track of the 
time that's gone by,
where we initially started, or where
we're supposed to end up.
So we choose to use a linear model 
sometimes in the classroom
such as a number line to help us with this.
Let's look at a problem.
Let's look at how we can use both a
clock and a number line to solve a problem.
Mike started reading at 8:30 p.m.
and read for two and a half hours.
At what time did Mike stop reading?
We can first use our clock by setting it to 8:30.
And then add our two and a half hours.
One hour.
Two hours.
And a half-hour.
So Mike stopped reading at 11 o'clock.
We can also use an open number line
to show this.
If Mike started reading at 8:30.
I can do a jump of an hour...
...to land at 9:30.
Another hour to show 10:30.
And then jump 30 minutes...
...to 11 o'clock.
Both showing that Mike stopped reading
at 11 o'clock.
While both tools work, the linear model 
helps us keep track of the
time that has gone by.
Now let's take a look at another problem.
Sanjay ended practice at 7:15 and
started practice at 5:30.
How long was Sanjay's practice?
One way we can show this is by starting our
number line at his start time of 5:30.
And his end time at 7:15.
We can jump forward to figure out the 
elapsed time.
So, I'm going to jump one hour
from 5:30 to 6:30...
...a half hour from 6:30 to 7:00...
...and then 15 minutes from 7 to 15.
I can then add all three times up to come 
up with my elapsed time.
1 hour.
1 hour and 30 minutes.
1 hour and 45 minutes.
So Sanjay's practice was 1 hour and 45
minutes.
I can also look at this another way by 
jumping backwards.
So, I can look at my end time of 7:15 and
start there and jump back to 5:30.
I could jump back one entire hour to 6:15.
Jump back 15 minutes...
...to 6 o'clock.
And then jump back 30 minutes to 5:30.
I can then add up all my times again to 
come with 1 hour and 45 minutes.
As you can see the number line us to
easily keep track of
the time that has gone by.
While both the clock and the number line work,
a number line is easy to draw and 
always available.
Now let's take a look at one last problem.
A train leaves Chicago at 4:13
and travels for 6 hours and 28 minutes
to St. Louis.
At what time does it arrive in St. Louis?
So, we can start on our number line at 4:13.
And take a giant jump of 6 hours.
To 10:13.
This would have taken much longer to do 
on an analog clock.
Then we need to add our 28 minutes.
So I'm going to add 20 minutes...
...to end up with 10:33...
...end then do 8 jumps.
So, that ends up being 34, 35, 36, 37, 38, 
39, 40, 41.  10:41.
The train arrived at 10:41.
This has been a look at elapsed time 
on a number line.
See you next time on Math in Minutes.
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