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?
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!