Adam's average speed = 40 miles per hr
time for the trip = 30 minutes = 0.5 hrs
Distance travelled by adam = speed * time = 40 * 0.5 = 20 miles
Distance travelled by Beth is 5 miles longer means 20 + 5 = 25 miles
Time took by Beth is 20 minutes more means = 30 + 20 = 50 minutes = 0.83 hrs
Average speed of Beth = distance / time = 25 / 0.8333
Average speed of Beth = 30 miles : Answer
Hope it will help :)