Your child writes that 1.5 hours means 1 hour 50 minutes. The digits look convincing, but the conversion has treated an hour as though it contained 100 minutes.
The useful starting point is simple: 1 hour = 60 minutes. The decimal part describes a fraction of those 60 minutes. Half an hour is 30 minutes, so 1.5 hours is 1 hour 30 minutes.
This guide gives parents and students a short way to work through time questions and check the answer. The examples are original practice questions, not official examination questions. Use the parts that match what your child has already learnt.
First decide: a clock time or a duration?
A clock time tells us when something happens: 14:40 is a time of day. A duration tells us how long it lasts: 1.25 hours is an amount of time.
Before calculating, ask your child to label each piece of information. A question might give a starting time and a duration, then ask for a finishing time. Another might give two clock times and ask how much time passed. Those are different tasks.
Keep the notation clear in the working. Write “1 h 30 min” or “1.5 h” for that duration. A clock reading such as “1:30 pm” answers a different question.
Turn the decimal part into minutes
For a duration written in decimal hours, multiply the number of hours by 60 to get minutes. You can also keep the whole hours and convert just the decimal part.
1.5 hours = 1 hour + 0.5 hour.
0.5 × 60 = 30 minutes.
So 1.5 hours = 1 hour 30 minutes, or 90 minutes.
A few familiar fractions provide useful checks:
| Decimal hours | Part of an hour | Minutes |
|---|---|---|
| 0.25 h | One quarter | 15 min |
| 0.5 h | One half | 30 min |
| 0.75 h | Three quarters | 45 min |
| 0.2 h | One fifth | 12 min |
The last row is a useful test of understanding. A child who reads 0.2 hours as 20 minutes can check that one fifth of 60 is 12. Ask what fraction the decimal represents, then convert that fraction of an hour.
To go the other way, divide the number of minutes by 60. For example, 90 minutes = 90 ÷ 60 = 1.5 hours. Dividing by 100 would give 0.9 hours, which is less than an hour and cannot represent 90 minutes.
A finishing-time example
A journey starts at 14:40 and lasts 1.25 hours. At what time does it finish?
- Convert the duration. 0.25 × 60 = 15 minutes, so 1.25 hours is 1 hour 15 minutes.
- Add the whole hour. 14:40 + 1 hour = 15:40.
- Add the remaining minutes. 15:40 + 15 minutes = 15:55.
Check backwards: from 14:40 to 15:40 is one hour, and from 15:40 to 15:55 is another 15 minutes. The total matches the stated duration. A proposed answer of 16:05 would describe 1 hour 25 minutes instead.
For elapsed time, count across the hour
Suppose an activity starts at 10:45 and ends at 12:20 on the same day. Your child can count in three manageable steps:
- 10:45 to 11:00: 15 minutes.
- 11:00 to 12:00: 60 minutes.
- 12:00 to 12:20: 20 minutes.
Total: 15 + 60 + 20 = 95 minutes, or 1 hour 35 minutes.
A timeline is one way to show those jumps. Use a method your child understands or the method their teacher has taught. Avoid treating the clock readings as ordinary decimal numbers: the change from one hour to the next involves 60 minutes, not 100.
Try three questions before checking the answers
- Convert 1.2 hours to hours and minutes.
- Write 105 minutes in decimal hours.
- A practice session starts at 09:50 and lasts 45 minutes. What time does it end?
Show the practice answers
1. 0.2 × 60 = 12, so 1.2 hours = 1 hour 12 minutes.
2. 105 ÷ 60 = 1.75 hours. Check: 1 hour plus three quarters of an hour is 60 + 45 = 105 minutes.
3. Ten minutes takes us from 09:50 to 10:00. Another 35 minutes takes us to 10:35.
Check what the answer represents
MOE's primary Mathematics syllabus includes checking whether answers make sense in the context of a problem. For time questions, our suggested check is: Have I answered when, or how long? Are my units correct? Does the duration fit between the two times?
This is a practice routine, not an official marking rule. Follow the question's requested format and your teacher's instructions. If your child needs help, give them one conversion to explain before adding a longer word problem; our one-hint-at-a-time homework guide offers prompts to try.
If the same difficulty keeps returning, keep the attempted working for a conversation with their teacher. You can also explore Math support at Genie and arrange a free trial through the form below.