Question

A car covers 30km at a uniform speed of 30 km/h what should be its speed for the next 90 km if the average speed of the entire journey is 60 km/h

Answer 1

Distance traveled by car = 30 km
Speed of the car = 30 km/h
Time taken by the car = Distance/speed = 30/30 = 1 h

The average speed of the car = total distance/total time
Total distance = 30 km + 90 km = 120 km
Total time = 1 h + time taken to cover next 90 km
Let the time taken to cover next 90 km = x h

Average speed = (30 + 90)/(1 + x)
                           = 120/(1 + x) km/h
Given average speed = 60 km/h
⇒ 60 = 120/(1 + x)
60 + 60x = 120
60x = 60
x = 1 h

The time taken to cover next 90 km = 1 h
Speed for next 90 km = Distance/time 
                                      = 90/1
                                      = 90 km/h

Hope this helps you.