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Thursday, June 14, 2007

Brain teaser...
A plane maintains an average speed of 696mph from London to New York. It then returns from New York to London at an average speed of 145mph. What is the average speed for the entire journey?

4 comments:

  1. average speed = 240 mph

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  2. How's that, then? -confused

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  3. Distance = speed x time

    Average speed over = s1 = 696
    Average speed back = s2 = 145

    So if time over is t1 and time back is t2 then distance = s1 x t1 = s2 x t2

    Total average speed = (2 x distance)/(t1 +t2) = (2 x s2 x t2)/(t1 + t2) = (2 x s2)/(t1/t2 + 1) = (2 x s2)/(s2 / s1 + 1) = (2 x s2 x s1)/(s2 + s1) = (2 x 145 x 696)/(145 + 696) = 240

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