D = rt Uphill d = 10(12/60) d = 2 miles (12/60) represents part of an hour
Downhill 2 = 30t 2/30 = t 1/15 = t 1/15 of an hour = 4 minutes
Average speed = total miles covered/total time total distance = 4 miles total time = 4 minutes + 12 minutes = 16 minutes/60 = 4/15 of an hour 4/(4/15) = 15 mph