Average speed

[derrynoose]

A car travels up a mile-long hill at 30 miles mph. At what speed should it travel on the return journey so that the average speed for the whole journey is 60 mph?

[Tichfield]

If I have understood this correctly:-

There is no answer to this.

If the car has to travel up and down the hill at an average speed of 60mph, it has to travel 2 miles at 60 mph which would take 2 minutes.

But it has already taken 2 minutes to go up the hill - one mile at 30 mph takes 2minutes.


Maybe I'm wrong though.

[novalis]

It's an old question and Tichfield is right. It can't be done.

[jstjohn]

I reckon it cannot be done unless the car travels at infinite speed on the return journey.
The time required for a 2 mile journey averaging 60 mph is 2 minutes. However, 2 minutes has already been used in traversing the first leg of 1 mile at 30mph!

Can you fault the logic?