You need to go from point A to point B, and then back to point A. Points A and B are 20 miles apart. You go to point B at a constant speed of 15 miles per hour. If you want your overall speed to be 30 miles per hour, how fast would you have to go from point B back to point A?
From A to B and back the way is 40 miles long.
If the average speed is 30 miles/hour, the 40 miles must be taken is 1 hour 20 mimutes, or 4/3 hours. Since the first 20 miles take all the 4/3 hours to travel, the other 20 miles must be taken in no time.