Understand the idea.
Jump to a landmark
Elapsed time is the duration from the start of an event to its finish. Count forward using a whole hour as a landmark. From 9:45 to 10:00 is 15 minutes; from 10:00 to 10:20 is another 20 minutes.
Add the jump lengths to find minutes. Do not subtract clock digits as ordinary decimal numbers: the minute count restarts after 59 because an hour contains 60 minutes.
Choose the direction
Choose your direction from what is missing. A start plus a duration gives the finish, so count forwards. A finish with a duration gives the start, so count backwards. Split the duration at an easy hour boundary.
Keep the am or pm label and check any crossing of noon or midnight. To check a backwards answer, count forward from your proposed start for the full duration and see whether you reach the stated finish.
| Event | Time |
|---|---|
| Reading begins | 9:10 am |
| Reading ends | 9:45 am |
| Recess begins | 10:40 am |
See the method in action.
Across an hour
A swim runs from 10:35 am to 11:15 am. Find its duration.
Worked solution
- 1
Count up to the next hour.
From 10:35 am to 11:00 am, 25 minutes complete the hour.
- 2
Count from the hour to the finish.
From 11:00 am to 11:15 am is a further 15 minutes.
- 3
Add the two elapsed parts.
The two jumps together cover the whole swim without overlap.
40 minutes.
Working backwards
A 50-minute club ends at 4:20 pm. Find its start time.
Worked solution
- 1
Count back to 4:00 pm.
The finish is 4:20 pm, so the first 20 minutes backwards reach an easy hour landmark.
- 2
Find how much farther to go back.
The club lasted 50 minutes altogether, and only 20 have been counted so far.
- 3
Move back the remaining half hour.
Half an hour before 4:00 pm is 3:30 pm, the club start.
- 4
Check the forward duration.
3:30 to 4:00 takes 30 minutes, then 20 more reaches the stated 4:20 pm finish.
3:30 pm.
Try it yourself.
Check your understanding
A film begins at 2:45 pm and lasts 30 minutes. When does it finish?
Show answer & reasoning
3:15 pm.
From 2:45 pm, 15 minutes reaches 3:00 pm. Another 15 minutes completes the 30-minute film, so it finishes at 3:15 pm: minutes.
Keep practising
6 worksheet questions · 8 marks
Read the questions online
- A lesson runs from 9:15 am to 9:50 am. Find its duration.1 mark
- A walk runs from 11:40 am to 12:10 pm. Find its duration.1 mark
- A game starts at 3:25 pm and lasts 45 minutes. Find the finish time.1 mark
- A bus arrives at 5:05 pm after a 30-minute trip. Find its departure time.1 mark
- A trip starts at 8:50 am and ends at 10:15 am. Find its duration.2 marks
- Ella must arrive by 2:00 pm. Her walk takes 25 minutes. What is the latest time she can leave?2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- A lesson runs from 9:15 am to 9:50 am. Find its duration.1 mark
minutes.
35 minutes. - A walk runs from 11:40 am to 12:10 pm. Find its duration.1 mark
20 minutes to noon plus 10 minutes after noon gives 30.
30 minutes. - A game starts at 3:25 pm and lasts 45 minutes. Find the finish time.1 mark
35 minutes reaches 4:00 pm, then another 10 reaches 4:10 pm.
4:10 pm. - A bus arrives at 5:05 pm after a 30-minute trip. Find its departure time.1 mark
Back 5 minutes to 5:00 pm, then back 25 to 4:35 pm.
4:35 pm. - A trip starts at 8:50 am and ends at 10:15 am. Find its duration.2 marks
10 minutes to 9:00, then 60 to 10:00, then 15 more.
85 minutes equals 1 hour 25 minutes.
1 hour 25 minutes. - Ella must arrive by 2:00 pm. Her walk takes 25 minutes. What is the latest time she can leave?2 marks
Count backwards 25 minutes from 2:00 pm.
1:35 pm.
Curriculum reference & lesson scope
Australian Curriculum v9 / SA Curriculum Mathematics 2026
am/pm; unit conversions; start/finish/elapsed time.
Official curriculum source ↗