Question

a car completes a journey at an average speed of 40km/h. at what speed must it travel on the return journey if the average speed is 60km/h?

Answer 1

Required formula:

Average speed = Total Distance / Total Time  

⇒ Total time = total distance / average speed

The time taken for the out journey = [d/40] hr

The time taken for the back journey = [d/x] hr

And,

The time taken for the complete journey = [2d/60] hr

Now, we can write the equation as,

[d/40] + [d/x] = [2d/60]

⇒ d[(x + 40) / (40x)] = [2d/60]

⇒ x + 40 = [2*40x] / 60

⇒ 60x + 2400 = 80x

⇒ 20x = 2400

⇒ x = 120 mph