Position and Motion
A car travels from town A to town B.It first moves at a speed of 60 km/h for 20 minutes and then at 80 km/h for the rest of time.What is the car's average speed if the distance between town A and town B is 60 km?
- 魏王將張遼Lv 71 十年前最愛解答
First of all, after 20 minutes, the car has travelled 60 x (20/60) = 20 km.
So at that time the car still has to travel 40km.
If the car travels at 80 km/h for the remaining distance, it needs a further 40/80 = 0.5 hours or 30 minutes to finish the remaining 40km.
So to speak, it has totally used 50 minutes or 5/6 hours to travel the whole journey of 60km.
Hence its average speed = 60/(5/6) = 72 km/h資料來源： My Maths knowledge.