Time Calculations Using Arithmetic
Time uses base-60 (not base-10) which makes arithmetic with it slightly different from ordinary numbers. Once you know the conversions, the rest is straightforward.
Key Conversions
| Relationship | Conversion |
|---|---|
| 1 minute | 60 seconds |
| 1 hour | 60 minutes = 3,600 seconds |
| 1 day | 24 hours |
| 1 week | 7 days |
| 1 year | 365 days (366 leap year) |
Adding Time
Example: 2 h 45 min + 1 h 30 min
Minutes: 45 + 30 = 75 = 1 h 15 min. Hours: 2 + 1 + 1 (carry) = 4. Answer: 4 h 15 min
Subtracting Time
Example: 3 h 20 min − 1 h 50 min
Borrow 1 h: 80 min − 50 min = 30 min. Hours: 2 − 1 = 1. Answer: 1 h 30 min
Calculating Duration
Train leaves 09:25, arrives 12:10. Duration = 2 h 45 min
Multiplying and Dividing Time
A task takes 35 minutes; 6 tasks take 35 × 6 = 210 minutes = 3 h 30 min
Divide 2 h 30 min equally among 5: 150 ÷ 5 = 30 minutes each
Key Takeaways
- Time uses base 60 for seconds and minutes; base 24 for hours.
- When adding minutes exceeds 60, convert the excess to hours.
- Convert to a single unit (minutes or seconds) before multiplying or dividing.
Practice Questions
- Add 1 h 35 min + 2 h 50 min.
- Subtract 4 h 10 min − 1 h 45 min.
- A film starts at 14:25 and lasts 2 h 18 min. What time does it end?
- A machine produces 1 item every 8 minutes. How many items in 6 hours?
- How many seconds are in 3 hours 15 minutes?