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.