Suppose you travel to a certain destination at an average speed of 30 miles per hour.

Question

What must your average speed be on the return trip to bring your overall average speed for the round trip to 40 miles per hour?

Answer 1

at first glance you might think 50, since 50+30 divided by 2 = 40

but the correct answer is 60 mph on the return trip

d=rt

distance = speed times time traveled

same distance each way means

d =30t1 = 60(t2) where t2 is less than t1

t1/t2 = 60/30 = 2

t1/t2 = 2

if t2= 2 hours, then t1 = 4 hours

and let d = 120 miles

average speed is d/t = 120/4 =30 one way, 120/2 = 60 return trip

total distance divided by total time = average speed =40

120(2) divided by 2+4= 240/6 = 40 mph