An train travels 10 miles at a speed of 50 miles per hour. How fast must the train travel on the return trip if the round-trip travel time is to be 20 minutes?

We know speed = distance/time
Given distance = 10 miles
Speed = 50 miles per hour
Time = distance/speed = ⅕ hours = 12 min
Total time = 20 min
Time left = 20 – 12 = 8 min = 2/15 hour

So the train has to cover 10 miles in 8 minutes.
Speed = distance/time
= 10/(2/15)
= 10(15/2)
= 75 miles per hour

Hence option b is the answer.