posted by sam .
Mark averaged 60 miles per hour during the 30-mile trip to college. Because of heavy traffic he was able to average only 40 miles per hour during the return trip. What was Mark's average speed for the round trip?
time on first part = 30/60 = 1/2 hrs
time for return trip = 30/40 = 3/4 hrs
total time = 5/4 hrs
total distance = 60 miles
avg speed = 60/(5/4) = 48 mph