How to see it
Three quantities, one triangle. Distance on top, speed and time below. Cover the one you want. Distance is speed times time. Speed is distance divided by time. Time is distance divided by speed. Time must be in hours if the speed is in kilometres an hour.
A worked example
20 minutes is a third of an hour, so 2 hours 20 is 2 and a third hours. 45 times 2 is 90. A third of 45 is 15. 105 kilometres. And for speed: 72 kilometres in 1 hour 30 is 72 divided by 1.5, 48 kilometres an hour.
The common mistake: writing 2 hours 20 as 2.2 hours
The trap is 2 hours 20 minutes as 2.2 hours. Twenty minutes is a third of an hour, not 0.2. That gives 99, not 105. And average speed is total distance over total time, not the average of two speeds.
Try one
72 kilometres in 1 hour 30 minutes. What is the speed?
- A55
- B48
- C54
- D108
Show the answerHide the answer
Right: B, 48