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Homework Help: Finding Average Speed

  1. Mar 21, 2010 #1
    1. The problem statement, all variables and given/known data
    A person travels by car from one city to another with different constant speeds between a pair of cities. She drives for 30.3 min at 85.2 km/h, 14.4 min at 91.1 km/h, and 48.9 min at 43.1 km/h, and spends 19.1 min eating lunch and buying gas. The distance between the initial and final cities along this route is 100.0165 km. Find the average speed for the trip. Answer in units of km/h.

    2. Relevant equations

    3. The attempt at a solution
    I added up all of the times (30.3 min+14.4 min+48.9 min) and got 93.6 min. I converted this to hours by dividing by 60 and got 1.56 hours. Then I divided 100.0165 by 1.56 hours and got 64.11314103. (I use the whole solution. I don't round unless I have to.) It was incorrect. If average speed equals total distance over total time, then what did I do wrong? I know I don't use that 19.2 min because it's just extra time that was spent eating and buying gas. I don't need that. So what am I doing wrong?
  2. jcsd
  3. Mar 21, 2010 #2
    Don't forget the time spent getting gas (speed = 0). You'll have to include that
  4. Mar 21, 2010 #3
    you do need that, because during that time, the person traveled at a speed of zero.
  5. Mar 21, 2010 #4
    Oh, wait, I see what you're saying. Hold on, I'll try it. One sec. :3
  6. Mar 21, 2010 #5
    Yes, and he still spent an extra 19.2 mins on the trip. You can't just forget about that time.

    Just think about running around your block. If you run at a constant speed of 10km/hr, your average speed is obviously, 20km/hr. But if you stop for 30 seconds to take a breath halfway through, your average speed goes down.

    You must include the 19.2 mins
  7. Mar 21, 2010 #6
    Alright, I got it! Thank you very much!
  8. Mar 21, 2010 #7


    Staff: Mentor

    That's not at all obvious!:wink:
  9. Mar 21, 2010 #8
    lol my mistake. obv meant 10km/hr
  10. Mar 21, 2010 #9
    Ha ha, I kinda did a double take when I saw that too, but I knew what you meant, obviously. :wink:

    But I got the question right! Thanks again for the help! o:)
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