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Riddle Clarification

  1. Dec 22, 2006 #1
    A friend of mine asked the following riddle.

    2 mile stretch of road and you average 30MPH for the first mile, how fast do you need to travel for the 2nd mile to average 60 MPH over the entire trip. This would seem simple enough not taking into consideration time.

    Is it true that since the first 1 mile stretch used up 2 mins that it would be impossible to average 60 mph over the entire 2 miles??
     
  2. jcsd
  3. Dec 22, 2006 #2

    Doc Al

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    Staff: Mentor

    Right. To average 60 mph means you must traverse the full two-mile distance in two minutes. But if you just wasted the entire two minutes traveling the first mile... no way to make up for that. (It would require infinite speed!)
     
  4. Dec 22, 2006 #3
    You can also make this intuitive reasoning: assume, for simplicity, it's not 1+1 mile but 30 mile + 30 mile instead. If you average at 30MPH in the first 30 mile, you need 1 hour. To average 60MPH over the entire 60 mile trip, you have to make the 60 mile in 1 hour, but you have already spent that hour it in the first part...
     
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