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Treebeard's Stumper Answer
24 Jan 97

Here are two DVT quickies involving Distance, Velocity, and Time. Be careful, these are stumpers!

  1. Traffic is bad, so I only average 30 miles per hour driving to school. But I drive home at a steady 60 miles per hour. What's my average speed for the round trip?

  2. Traffic is bad again, so I drive to school at 30 miles per hour. How fast do I have to drive home to average 60 miles per hour for the round trip?

Distance doesn't matter for these, so pick an easy one. Say it's 60 miles to school and 120 miles both ways. Then:

  1. It takes 2 hours to get there at 30 mph, and 1 hour to get home at 60. So my average speed is 120 miles / 3 hours = 40 mph. Getting there takes longer, which skews the average.

  2. Light speed isn't fast enough for me to average 60 mph! It takes 2 hours to get there at 30 mph. But to average 60 for the round trip, I have to get there and back in 120 miles / 60 mph = 2 hours. So I've already used up all my time.

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Marc Kummel / mkummel@rain.org