Mathematics (Core)Exam code: 0580
Compound Measures
Number
The relationship
- Speed is distance divided by time, and in this context "speed" always means average speed
- The three arrangements follow from one another
| To find | Calculate |
|---|---|
| Speed | distance ÷ time |
| Distance | speed × time |
| Time | distance ÷ speed |
- Common units are metres per second (m/s) and kilometres per hour (km/h)
- A journey made in stages needs total distance divided by total time, not an average of the separate speeds
Time in hours
- Time must be expressed in the unit the answer requires, and part-hours are decimals of an hour rather than minutes written after a point
- 1 hour 48 minutes is 1.8 hours, because 48 minutes is 48/60 of an hour
- Writing 1.48 instead is a common error in these questions
Worked example
Average speed with a part-hour
A car travels 153 km in 1 hour 48 minutes. Find its average speed in km/h.
Solution:
- Convert the time to hours: 48 ÷ 60 = 0.8, so the time is 1.8 hours
- Speed is distance divided by time: 153 ÷ 1.8
- The average speed is 85 km/h
- Check by reversing: 85 × 1.8 = 153