Speed, Distance and Time
Primary 6 · Mathematics
Speed, Distance and Time
Lesson 5 of 5
15 min
Let's begin
Habimana travels from Kigali towards Musanze. He goes 60 km by bus in 1 h 30 min, waits 30 minutes at a stop, then rides 30 km more in 1 h. The bus did not keep one steady speed. So what speed describes the whole trip? That is the average speed.
Average speed = total distance ÷ total time
Habimana: total distance = 60 + 30 = 90 km. Total time = 1.5 h + 0.5 h (the stop) + 1 h = 3 h.
Average speed = 90 ÷ 3 = 30 km/h.
If a question asks only about the time spent moving, leave out the stop: 90 ÷ 2.5 = 36 km/h. Read the question carefully to know which one it wants.
Warning
Never just average the speeds. A cyclist rides 30 km to the market at 15 km/h and comes back the same 30 km at 10 km/h. It is tempting to say (15 + 10) ÷ 2 = 12.5 km/h, but that is wrong.
Time going = 30 ÷ 15 = 2 h. Time coming back = 30 ÷ 10 = 3 h.
Average speed = total distance ÷ total time = 60 ÷ 5 = 12 km/h.
The slow part lasted longer, so it pulls the average down.
Two buses start at the same time from towns 240 km apart and drive towards each other. One goes at 70 km/h and the other at 50 km/h. When do they meet?
Every hour, the gap between them shrinks by 70 + 50 = 120 km. Time to meet = 240 ÷ 120 = 2 h.
In that time the first bus travels 70 × 2 = 140 km and the second 50 × 2 = 100 km. Check: 140 + 100 = 240 km.
If one traveller starts earlier, first work out how far that one goes alone. Then use the combined speed for the rest of the gap.
A bus leaves at 07:00 and arrives at 09:30 after 150 km. Time = 2 h 30 min = 2.5 h. Average speed = 150 ÷ 2.5 = 60 km/h.
Tip
Average speed = ALL the distance ÷ ALL the time. Add the distances, add the times, then divide once.
Answers
What you learned today
Sources and credits
Written by Ganzaa.
Quick check
A car travels 90 km in 1 h, then 60 km in 2 h. What is its average speed for the whole trip?
