A plane maintains an average speed of 696mph from London to New York. It then returns from New York to London at an average speed of 145mph. What is the average speed for the entire journey?
240mph: let the distance be x miles. Therefore, the outward trip takes x / 696 hours and the return trip takes x / 145 hours. The total distance travelled is hence 2x, in a time of (x / 696 + x / 145). Therefore the speed (distance / time) is given by 2x / (x / 696 + x / 145) = 240 mph. QED.