Duration — Deep Dive Learning

Calculate how long things take using start and end times

What is duration?

  • Duration is how long something lasts.
  • We measure duration in minutes, hours, or days.
  • To find duration, subtract the start time from the end time.
  • We use duration to plan our day, cook, travel, and more.
  • Example: A film starts at 7:00 and ends at 9:30. Duration = 2 hours 30 minutes.

Counting on in minutes

  • One way to find duration is to count on from the start time.
  • Start: 2:15. End: 2:45.
  • Count from 2:15 to 2:45 = 30 minutes.
  • Start: 3:00. End: 3:55.
  • Count from 3:00 to 3:55 = 55 minutes.

Counting over the hour

  • Start: 2:45. End: 3:30.
  • Step 1: Count from 2:45 to 3:00 = 15 minutes.
  • Step 2: Count from 3:00 to 3:30 = 30 minutes.
  • Total: 15 + 30 = 45 minutes.
  • This method works for any times that cross the hour.

Hours and minutes together

  • Start: 10:00. End: 12:30.
  • Step 1: Hours from 10 to 12 = 2 hours.
  • Step 2: Minutes from :00 to :30 = 30 minutes.
  • Total duration: 2 hours 30 minutes.
  • Always state hours first, then extra minutes.

Worked Examples

Example 1: Cooking time

  1. Put the pie in at 1:20 PM.
  2. Take it out at 2:05 PM.
  3. Count: 1:20 to 2:00 = 40 minutes.
  4. Then 2:00 to 2:05 = 5 minutes.
  5. Total cooking time: 45 minutes.

Example 2: Working shift

  1. Start: 9:00 AM. Finish: 1:30 PM.
  2. Hours: 9 to 1 = 4 hours.
  3. Minutes: :00 to :30 = 30 minutes.
  4. Total shift: 4 hours 30 minutes.

Practice Problems

  • A film starts at 6:00 PM and ends at 8:15 PM. How long is it?
    Hint: Count the hours, then the minutes.
    Answer: 2 hours 15 minutes
  • You start cooking at 11:45 AM and finish at 12:30 PM. How long did it take?
    Hint: Count from 11:45 to 12:00, then to 12:30.
    Answer: 45 minutes
  • Work starts at 8:30 AM and ends at 4:00 PM. How long is the shift?
    Hint: Count hours from 8 to 4, then adjust for the :30.
    Answer: 7 hours 30 minutes

Challenge Question

A train leaves at 09:47 and arrives at 11:23. It stops for 8 minutes at one station. How long is the actual travelling time?

Hint: First find the total journey time, then subtract the stop.

Answer: 1 hour 28 minutes

Total time: 09:47 to 11:23 = 1 hour 36 minutes. Subtract 8 minutes stop = 1 hour 28 minutes travel.

Common Mistakes to Avoid

  • Mistake: Subtracting times like normal numbers: 3:10 - 2:50 = 0:60
    Correction: 3:10 - 2:50 = 20 minutes
    Why: Count forward from 2:50 to 3:10. That is 10 min to 3:00, then 10 more = 20 min.
  • Mistake: Forgetting that 1 hour = 60 minutes (not 100)
    Correction: 1:30 to 2:00 is 30 minutes, not 70 minutes
    Why: Hours wrap at 60 minutes, not at 100.
  • Mistake: Getting confused when times cross the hour
    Correction: Break it into parts: time to the next hour, then remaining time
    Why: From 10:45 to 11:20: 15 min to 11:00, then 20 min = 35 min total.
Start the interactive Duration lesson →