# math

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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

• math -

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

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