A motorist travels 90 miles at a rate of 20 miles per hour. If he returns the same distance at a rate of 40 miles per hour, what is the average speed for the entire trip, in miles per hour? (Explain)

  26 2/3

Step-by-step explanation:

Average speed is total distance divided by total time.

The time for the first trip was ...

  (90 mi)/(20 mi/h) = 4.5 h

The time for the return trip was ...

  (90 mi)/(40 mi/h) = 2.25 h

Then the average for the trip and return is ...

  total miles/total time = (90 +90)/(4.5 +2.25) = 26 2/3 . . . . mi/h

The average speed for the round trip was 26 2/3 miles per hour.