A man rides a bike from melbourne to Koo-wee-rup at a speed of 20km/h. He immediately comes back by train, averaging 60km/h. What is his average speed for the entire jourmey from melbourne to Koo-wee-rup and back??

Use:

Speed = \frac{Distance}{Time}

Let the distance to be x. The total distance becomes 2x.

The time taken is given by distance divided by speed.