math
posted by Anonymous .
you go up a hill at 10 miles per hour and down the hill at 30 miles per hour what is the average speed and it's not 20 miles per hour

time to go up hill = d/10
time to go down = d/30
total time = d/10 + d/30 = 4d/30
avg speed = total distance / total time
= 2d/(4d/30)
= 2d(30/4d)
= 60/4 = 15 mph