Use speed = distance ÷ time in all three arrangements, with hours, minutes and unit changes, and find average speeds over several stages.
What you will learn
- Find speed, distance or time
- Work in hours and minutes
- Change km/h to m/s
- Find an average speed for a whole journey
One formula, three ways round
speed = distance ÷ time
distance = speed × time
time = distance ÷ speed
Keep the units matched: km and hours give km/h. 40 minutes is 40/60 = 2/3 of an hour, not 0.4 hours. So change minutes to hours (÷ 60) before you use the formula.
- a cyclist rides at 18 km/h for 50 minutes: how far?
- 50 minutes = 50/60 = 5/6 of an hour
- distance = speed × time = 18 × 5/6 = 15 km
When the time comes out in hours, multiply by 60 to get minutes. 39 km at 45 km/h takes 39 ÷ 45 = 0.8666... hours, and 0.8666... × 60 = 52 minutes. To find an arrival time, add that time to the start: leaving at 10:15, the cyclist arrives 52 minutes later, at 11:07.
Two people moving towards each other close the gap by the sum of their speeds. If they start 100 km apart at 30 km/h and 20 km/h, the gap shrinks by 30 + 20 = 50 km each hour, so they meet after 100 ÷ 50 = 2 hours.
The rest of this lesson and its 7 practice questions are locked.
In this lesson
- One formula, three ways round
- Changing units, and average speed
- Try it: Check a conversion
7 practice questions, marked as you go; about 25 minutes in all.
Key terms
Towards each other
In this unit: Rates, compound measures and growth
Speed, density and pressure, rates of change read from graphs, compound interest and depreciation, and exponential growth and decay.