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Homework Help: Avg speed help

  1. Aug 27, 2007 #1
    ok heres the question then what i did so far.

    Q> calculate the avg speed of a complete round trip in which the first 200km is covered at 90km/hr, followed by a one-hour lunch break, and final 200km is covered at 50km/hr.

    What i tried> 200km/90kmphr then i got 2.2hr and then 200/2.2hr to get the avg speed, which was 90km.

    then i did 200km/50kmphr then i got 4hr and then 200km/4hr and got 50km. then avg those speeds but i keep geting around 70 and the answer is supposed to be 55km/hr.

    what am i doing wrong???
  2. jcsd
  3. Aug 27, 2007 #2
    "and then 200/2.2hr to get the avg speed, which was 90km."

    but you are missing first half with the second half?

    average speed = total distance travelled/total time

    so total dist = 400
    and total time =2.2+4+1
  4. Aug 27, 2007 #3
    aaaa i see thank you
  5. Aug 27, 2007 #4
    ohh.. now I got it, what you were doing there.. "200/2.2"
    I think it would have worked if they hadn't stopped for the break
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