Lesson Video: Start Times to the Nearest Five Minutes Mathematics

In this video, we will learn how to find the duration of events on analog clocks and number lines.

09:22

Video Transcript

Start Times to the Nearest Five Minutes

In this video, we will learn how to count in fives on a clock or a number line to find the duration of events in minutes.

These children are enjoying their morning break time. Their break time starts at 11 o’clock, and their morning break time ends at 11:15. How long did that break time last?

Did you notice that the minute hand on the clock started at number 12? This means it’s something o’clock. Then, it moved around the clock to number three, which is quarter past 11 or 11:15. We can shade the clock to show how many minutes have passed from 11 to 11:15. We need to count in fives because each of the numbers on the clock is worth five minutes.

So, from 12 to one, there are five minutes. And from one to two, there are five minutes. And from the numbers two to three, there’s another five minutes. So, let’s count in fives: five, 10, 15. So, morning break time lasts for 15 minutes. The duration of morning break time is 15 minutes. So, the duration means how long morning break time lasts. If morning break time starts at 11 and finishes at 11:15, its duration is 15 minutes.

If afternoon break time ends at 2:15 or quarter past two and lasts for 15 minutes, what time does afternoon break start?

In this problem, we know the end time, but we don’t know the start time. We know that break time ends at 2:15, but we also know that break time lasts for 15 minutes. So, we could start at 2:15 and count back 15 minutes on our clock face. Five, 10, 15. The minute hand started at number three and moved back to number 12. Now, the clock says 2 o’clock. If afternoon break ends at 2:15 and lasts for 15 minutes, then break time starts at 2 o’clock. We counted back from 2:15 to 2 o’clock in five-minute intervals. We could also use a number line to help us solve the problem. We know that break time ends at 2:15 and lasts for 15 minutes. So, we can start at 2:15 on our number line and count back in fives. Five, 10, 15 minutes.

Let’s practice finding the start time when we’re given the duration of an event and the end time.

It is 9 o’clock. Pick the time it was one hour ago. Was it 8 o’clock, 9 o’clock, or 10 o’clock?

We’re shown two different clocks. This clock shows 9 o’clock. We have to work out what time it was one hour ago. So, we need to work out what the time was an hour before 9 o’clock. The hour before 9 o’clock is 8 o’clock. We know that nine take away one is eight. So, 9 o’clock take away one hour is 8 o’clock. Which of our three clocks shows this? It’s the first clock. If it’s 9 o’clock now, then the time one hour ago was 8 o’clock.

It is 35 minutes after nine. Pick the time it was 15 minutes ago. Was it 9:20, 9:15, or 9:50?

We’re shown two different clocks. The second clock shows the end time after 15 minutes have passed. But we don’t know the start time. We know that it’s 9:35 after 15 minutes have passed. So, to find the time 15 minutes ago, we need to count back from 9:35 15 minutes. If we count back five minutes, the time will be 9:30. And we count back another five minutes; it’s 9:25. We’ve counted back 10 minutes so far. And if we count another five minutes, we’ve counted back 15 minutes altogether. And the time on the clock now says 9:20. We know that 35 take away 15 is 20. So, 35 minutes take away 15 minutes is 20 minutes.

Which of our three clocks shows the time 9:20? If it’s 35 minutes after nine, the time 15 minutes ago was 9:20. To find the answer, we counted back three lots of five minutes because three fives are 15. The time 15 minutes ago was 9:20.

Mason wants to meet his friends at the park at 4 o’clock. It will take him 15 minutes to walk there. What time does he need to leave his house?

We know that Mason needs to be at the park at 4 o’clock, and we know it takes him 15 minutes to walk there. We could use a number line to help us solve this problem. We know that Mason needs to be at the park by 4 o’clock. Let’s call this the end time. We also know that his walk will take him 15 minutes. So, we need to use the number line to find out what time it was 15 minutes ago. We need to count back 15 minutes.

We know that halfway between 3 and 4 o’clock, it’s 3:30. Halfway between 3:00 and 3:30 is 3:15. And halfway between 3:30 and 4 o’clock, it’s 3:45 or quarter to four. So, let’s start at 4 o’clock and count back 15 minutes. Each of the smaller divisions on our number line is worth five minutes. If we count back five minutes, that will take us to 3:55 or five minutes to 4 o’clock. There are 60 minutes in an hour. So, if we take away five minutes, we’re at 3:55. If we count back five minutes from 3:55, that will take us to 3:50. Five minutes less than 3:50 is 3:45. So, Mason needs to leave his house at 3:45.

We can check our answer using a model of a clock. The minute hand started at number 12, and we had to count back 15 minutes or three lots of five: five, 10, 15. So, if Mason needs to meet his friends at 4 o’clock and it takes him 15 minutes to walk to the park, he will need to leave his house at 3:45.

What have we learned in this video? We have learned how to count in fives on a clock or a number line to find the duration of events in minutes.

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