Answer: option B is the correct answer.
Step-by-step explanation:
Susan drove at an average speed of 30 miles per hour for the first 30 miles of a trip.
Time = distance/speed
This means that time spent by Susan in travelling the first 30 miles would be
Time = 30/30 = 1 hour
He travelled at an average speed of 60 miles per hour for the remaining 30 miles of the trip.
Time = 30/60 = 0.5 hours
Total distance travelled is 30 + 30 = 60 miles.
Total time spent = 1 + 0.5 = 1.5 hours
Speed = distance/time
Average speed = 60/1.5 = 40 miles per hour.
