Video Transcript
End Times to the Nearest Five
Minutes
In this video, we’re going to model
how long something takes or its duration on analog clocks or number lines. And we’re going to count in fives
to find the end times of events in minutes.
Here’s a bowl of cookie dough. Let’s imagine that we’ve done all
the preparation and the mixing, and all that’s left now is to put the cookies in the
oven. When will our cookies be ready? In other words, what’s the end time
of the cooking activity? At what time will they have
finished being baked? To find the answer, we need to know
two things: we need to know the start time and the time taken.
In this example, when we say start
time, what we mean is the time now, the time at which we’re about to start
cooking. Let’s imagine that we look up at
the kitchen clock and we can see that the time is five minutes past 3. We know this because we can see
that the minute hand has made one jump of five minutes from an o’clock time. And because the hour hand looks
like it’s pointing to the number three, it must have only just been 3 o’clock. Our start time, then, is five
minutes past 3 or 3:05.
The second piece of information
that we need to work out an end time is how long something’s going to take. We can use a special maths word
here to help us, and that’s the word “duration.” Duration of something is just the
time that it takes. So, in this example, we need to
know the duration of baking the cookies. Now, let’s imagine that we’re
following a recipe card. And on the card, it says, “Cooking
time: 25 minutes.” So, we know the time it’s going to
take to bake our cookies is 25 minutes.
We have the two pieces of
information we need to be able to work out the end time when our cookies are going
to be ready. We’ve got the start time and the
time it is going to take. Now, how can we use these two
pieces of information to help us? One way to find the answer is to
use an analog clock to help. We can begin by showing our start
time, five minutes past 3. As we’ve said already, at 5 past 3,
the minute hand points to the number one because five minutes have gone by since an
o’clock time. But where’s the minute hand going
to be at the end of our cooking time? Where will it be in 25 minutes
time?
We can count in jumps of five
minutes to find out. Five, 10, 15, 20, 25. We’ve modeled the time that it
takes or the duration on our analog clock. And we found out that after 25
minutes have gone by, the minute hand will be pointing to the number six. This is going to be a half past
time, isn’t it? It’s going to be half past 3. So, we’d better draw the hour hand
in the right place too, halfway between the numbers three and four. The end time for baking our
cookies, in other words, when we’re going to open the oven and take them out, is
going to be half past 3.
Another way we could find the same
answer is by sketching a number line and using it to help. Do you remember how many minutes
there are in one hour? There are 60 minutes, aren’t
there? So, we can mark our number line
from zero to 60. Now, because we’re going to be
counting in fives here, we’d better mark all the halfway points too. There we go. Now, our number line is marked in
fives. Now, where are we going to start
counting from? Do you remember our start time? It was five past 3, wasn’t it? Or 3:05.
So, we need to start counting from
the number five on our number line. And just like with the analog
clock, we’re going to skip count in fives until we’ve counted on 25. Five, 10, 15, 20, 25. If we start with a time that’s five
minutes past something and we count on 25 minutes, then we end with a time that’s 30
minutes past the hour. That’s another way to find out that
the time we need to pull our cookies out of the oven is 3:30 or half past 3.
Let’s answer some questions now
where we have to put into practice what we’ve learned. In each question, we’re going to be
looking for the end time. And what are we going to use to
help? We’re going to use the time that
something starts and also how long it takes or the duration.
It is 2 o’clock. Pick the time it will be in one
hour. 1 o’clock, 3 o’clock, or 12:15.
To begin with, we’re told that the
time is 2 o’clock. How do we know this? Well, we can see that the minute
hand on our clock face is pointing right up to the number 12; this tells us it’s an
o’clock time. And then, the hour hand is pointing
straight at the number two; it’s 2 o’clock. And the question asks us to pick
what the time will be in one hour. Now, we’re given three different
times to choose from, and these aren’t just any three times. One of them is going to end up
being the right answer; we know that. But the other two clocks show times
that we could choose by accident if we made certain mistakes.
Let’s go through our possible
answers and keep an eye out for those mistakes. We don’t wanna make them, do
we? If we look at our first clock face,
we can see that it’s another o’clock time. The minute hand is pointing to the
12; this clock shows 1 o’clock. Now, this is one-hour difference
from 2 o’clock, isn’t it? Is this the right answer? It’s not, is it? We know that it’s 2 o’clock
now. So, 1 o’clock is an hour before
this time; it’s been and gone. It was 1 o’clock one hour ago. We need to find the time that’s one
hour in the future.
So this clock might be an easy
mistake to make if you’re counting in the wrong direction. But we’re not going to make that
mistake. We’re going to count onwards from 2
o’clock. In one hour’s time, the minute hand
will have gone all the way around the clock back to another o’clock time. But the hour hand will have moved
on to the next number. It’s going to be 3 o’clock. Now, here’s where we need to make
sure we don’t make a second mistake. We know that when it’s 3 o’clock,
one of the hands points to the number 12. That’s an o’clock time. And the other hand points to number
three, 3 o’clock.
Can you see what’s similar about
our two remaining answers? They both have hands pointing to
the 12 and to the three. It would be very easy to choose the
wrong clock here. Now, we’ve got digital times
written underneath each clock, so we can see which one is 3 o’clock. But even if the digital time wasn’t
there, we could tell which clock was correct by looking for the one where the minute
hand — that’s the longest hand — is pointing to the 12 and the hour hand — that’s
the shorter hand — is pointing to the three. On our last clock, those hands are
pointing the other way around, and so it shows 12:15 and not 3 o’clock. If it’s 2 o’clock, the time it will
be in one hour will be 3 o’clock.
It is 25 minutes after 9. Pick the time it will be in 10
minutes. 9:15, 9:30, or 9:35.
In this question, we’re being asked
to find the end time, in other words, the time that it’s going to be. And to help us, we’re told the time
that it is now, which is 25 minutes after 9, and also the duration or the time
that’s going to go by, which is 10 minutes. So, we’re being asked if it’s 25
past 9 now, what will the clock look like in another 10 minutes time? Let’s sketch some clocks to help us
here.
We know that when the time is 25
minutes after nine, the minute hand points to the number five and the hour hand is
almost halfway between the numbers nine and 10. This is our start time, and we also
know the time that goes by or the duration. The minute hand is pointing to the
number five, but 10 minutes need to go by. Remember, the gap between two
numbers that are next door to each other on a clock face is five minutes. So, we can skip count in fives to
count 10 minutes. Five, 10. After 10 minutes have gone by, the
minute hand is going to move so that it is pointing to the number seven.
Now, we know the end time we’re
looking for. The minute hand is going to be
pointing to the seven. And because it’s just past half
past nine, we’d expect the hour hand to be just a little bit more than halfway
between nine and 10. And if we look at our answers, we
can see that only one of the clocks looks the way we wanted to. It’s this one, isn’t it? We know that 25 plus 10 is 35. And so, 9:25 plus 10 minutes is
9:35. The time that it will be in 10
minutes is 9:35.
Emma leaves her house to walk to
the library at 10:30. The walk takes her 15 minutes. What time does she arrive?
This question describes Emma who’s
walking from her house to the library. And the question asks us to find
out what time she gets there. What time does she arrive? Now, we’re given two pieces of
information to help us. We’re told the start time of Emma’s
walk. She leaves her house at 10:30 and
we’re also told the duration or how long it takes her. The walk takes her 15 minutes. Now, we can use the start time and
the duration to help us find the end time. We need to start at 10:30 and count
on 15 minutes.
We could use a number line to help
us. We know that there are 60 minutes
in an hour, so we can label our number line from zero to 60. This shows a whole hour. Now, Emma starts walking at half
past 10 or 10:30. So, we can mark the start time on
our number line, 30 minutes past 10. Now, let’s show the duration. We need to count on 15 minutes, and
we’re going to do this by skip counting in fives. Five, 10, 15. How many minutes past 10 is it
now? We’ve ended up halfway between the
numbers 40 and 50. This shows the number 45, doesn’t
it? It’s 45 minutes past 10.
To work out the end time or the
time that Emma arrives at the library, we’ve used the time that she starts her
journey and how long it takes. If we count on 15 minutes from
10:30, the end time will be 10:45.
Jennifer leaves to take her dog for
a walk at 7:05. They walk for 20 minutes. What time do they return?
This question describes Jennifer
who’s taking her dog for a walk. And we’re asked what time they both
get back from their walk, in other words, what time their walk ends. And to help us know what time their
walk ends, we’ve got two pieces of information to help us. Firstly, we know what time their
walk starts. The start time for their walk is
7:05 or five past seven. This is going to be important to
us, so let’s sketch it on a clock. Now, we know at 7 o’clock, the
minute hand would be pointing directly to the number 12 and the hour hand would be
pointing straight at the number seven.
Now, at 7:05, five minutes have
gone by. And because we know the gap between
one number and the next number on a clock face is worth five minutes, we know the
minute hand at five past seven is pointing to the number one. And because it’s still quite near
to 7 o’clock, the hour hand won’t have moved very much. This is what the clock looks like
when Jennifer and the dog leave.
The other piece of information that
we’re going to use here to help us is how long it takes them. We’re told that they walk for 20
minutes. What will happen to the hands of a
clock in 20 minutes? At five past 7, the minute hand is
pointing to the number one as we’ve said. Let’s count on another 20
minutes. And we’re going to skip count in
fives to help us do this. Five, 10, 15, 20.
We can see then that when Jennifer
returns home, the minute hand is going to be pointing to the number five. We know that when it points to a
six, it is a half past time. And at that point, the hour hand
will be halfway between the seven and the eight. So, at the moment, it’s almost half
past 7. So, we could draw the hour hand
about here. So, how would we read this
time? We know that it’s still 7
something, that if the minute hand was pointing to the number six, it would be
7:30. At the moment, it’s one number
before the six, isn’t it? So, this is five minutes before
30. We know that 30 take away five is
25. And so, the time that Jennifer and
her dog return home is 7:25.
What’ve we learned in this
video? We’ve learned to model the time
that something takes or its duration on analog clocks or number lines. And we’ve skip counted in fives to
find the end times of events in minutes.
