Quick revision notes · Mathematics 4024 · Chapter 10 of 35
Time
Clocks, durations, decimal hours, timetables and time zones.
3 sectionsWorked examples
Common mistakes
At a glance
Clocks, durations, decimal hours, timetables and time zones.
112- and 24-hour clocks
- 24-hour times use four digits: 07 45, 19 45. Midnight is 00 00; noon is 12 00.
- pm → 24-hour: add 12 to the hours (except 12 pm). am times stay the same (12 am = 00 00).
- 24-hour → 12-hour: subtract 12 from hours above 12 and write pm.
| 12-hour | 24-hour |
|---|---|
| 12:30 am | 00 30 |
| 7:45 am | 07 45 |
| 12:15 pm | 12 15 |
| 7:45 pm | 19 45 |
| 11:59 pm | 23 59 |
Worked example
Write 4:05 pm in the 24-hour clock.
- pm and after 1 pm → add 12.
- 4 + 12 = 16.
16 05
2Time intervals and decimal hours
- Count up on a time line: to the next whole hour, then whole hours, then the minutes left.
- Never subtract times like decimals: 21 15 − 18 50 is not 2.65.
- Decimal hours → minutes: multiply the decimal part by 60. 2.35 h = 2 h 21 min.
- Minutes → decimal hours: divide by 60. 45 min = 0.75 h; 48 min = 0.8 h.
Time line method
- From the start time, count to the next o'clock.
- Count the whole hours.
- Add the minutes at the end.
- Add the pieces together.
Worked example
A bus leaves at 22 40 and arrives at 03 15 the next day. How long is the journey?
- 22 40 → 23 00: 20 min.
- 23 00 → 03 00: 4 h.
- 03 00 → 03 15: 15 min.
4 h 35 min
3Timetables and time zones
- Read a timetable along the row for the stop and down the column for one bus or train.
- Arrival = departure + journey time.
- A place that is ahead is later there: add the difference. Behind: subtract.
- Flight problems: work in one time zone first, then convert at the end.
Worked example
Karachi is 5 h ahead of London. A flight leaves London at 22 00 (London time) and takes 8 h. What is the local time in Karachi on landing?
- Landing in London time: 22 00 + 8 h = 06 00.
- Karachi is 5 h ahead: 06 00 + 5 h.
11 00 (next day)
Exam tip: Watch for journeys that cross midnight — the date changes too.
✗Common mistakes
| Don’t write | Write instead |
|---|---|
| 2.35 h = 2 h 35 min | 0.35 × 60 = 21 min → 2 h 21 min |
| 21 15 − 18 50 = 2 h 65 min | Count up: 2 h 25 min |
| 12:30 am = 12 30 | 12:30 am is just after midnight: 00 30 |
| Using 100 minutes in an hour | There are 60 |
★In the exam
- Show your time line — it earns marks even if you slip in adding.
- Change minutes to hours (÷ 60) before using them in a speed formula.
- Give answers in the form asked: 24-hour clock, hours and minutes, or just minutes.