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Homework Help: Finding average velocity

  1. Apr 24, 2013 #1
    1. The problem statement, all variables and given/known data

    A car travels at a speed v (km/h) for 40 minutes, then at a speed v/2 for 40 minutes, and finally at a speed v/3 for 20 minutes. What is the average speed for the trip

    2. Relevant equations

    avg v = displacement / time

    3. The attempt at a solution

    40v + 20v + 20/3 v = displacement
    time = 40+ 40+ 20 = 100minutes

    The answer given is 33.33
    but, how do you cancel out the v.. i'm confused :(
  2. jcsd
  3. Apr 24, 2013 #2
    the average velocity is (5/9)*v
  4. Apr 24, 2013 #3
    what are the units of the 33.33
  5. Apr 24, 2013 #4
    i dont remember :/ but in general, how to solve this question?
  6. Apr 24, 2013 #5
    you cant eliminate the v unless it gives you a value for it. the only thing i can think of is that 33.33 is the average time per journey of the car. if it is asking for the average speed then v must have been 60 km/h.
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