You drive to the city at an average speed of 40 km/h and return at an average speed of 60 km/h. Find your average speed for the entire trip? Explain why the answer is not 50 km/h.

Well, let's see:

If you live 'D' kilometers from the city, and you go and return by the same route, then

- - The time it takes you to reach the city, at 40 km/h, is D/40 hours.

- - The time it takes you to return, at 60 km/h, is D/60.

- - The total distance you cover is 2D, and the total driving time is

(D/40) + (D/60)

= (3D/120) + (2D/120)

= 5D/120 = D/24

- - Your average speed for the entire trip is

(distance covered) / (time to cover the distance)

= (2D) / (D/24)

= 48 km/hr.