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Homework Help: Average speed

  1. Feb 6, 2008 #1
    [SOLVED] Average speed

    Hi, Please could someone enlighten me as to how to solve the following, given that the total time of the journey is not given?

    A commuter drives to work 14 miles. Due to traffic, the commuter averages 25 miles per hour on the first 7 miles of the drive. How fast must the commuter go on average on the second half to average 40 miles per hour for the entire drive?

    Many thanks
  2. jcsd
  3. Feb 6, 2008 #2


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    Staff Emeritus
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    Note that 7 miles is half of 14 miles...
  4. Feb 7, 2008 #3
    Thank you for your reply. I had noticed that! However, given that 55mph is the wrong answer I still lack the tools to finish the job. Please could you tell me how to solve this, then I'll be equipped for life :)
  5. Feb 7, 2008 #4
    this should help ya....:

    Louise runs the first half of a race at 5 miles per hour. Then she picks up her pace and runs the last half of the race at 10 miles per hour. What is her average speed on the course?

    A The answer is not 7.5 miles per hour. Assume the course is 20 miles long. Louise needs 2 hours to run the first 10 miles (the first half of the course) and 1 hour to run the last 10 miles (the last half), meaning she needs 3 hours to run the total of 20 miles. Therefore, her average speed is 6.7 (or 6 2/3) miles per hour.
  6. Feb 7, 2008 #5
    Okay - So:

    7 miles at 25mph = 16min 48 secs

    14 miles at 40mph = 21 mins

    21mins - 16.8 mins = 4.2mins

    So the driver must do:

    7miles in 4.2 mins = 100 mph (99.99999)

    to average 40mph for the whole journey. Am I right?
  7. Feb 8, 2008 #6
    what do ya think...????
  8. Feb 8, 2008 #7
    I think I love ya... :0
  9. Feb 8, 2008 #8
    keep loving me...:p

    U are right dear...

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