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Thread: Calculating Average Speed,...for those who appreciate thinking.

  1. 20th June 2010, 10:43 AM #1
    RoverP6B
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    Calculating Average Speed,...for those who appreciate thinking.

    As we all I assume drive a car of some description, calculating average speed for a trip could be something that we might like to do every now and then.

    So here is a little question to think about and to post your answers to.

    "A driver sets out on a journey. For the first half of the distance she drives at the leisurely pace of 60km/h; she drives the second half at 120km/h. What is her average speed on this trip?"

    Ron.
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  2. 20th June 2010, 10:45 AM #2
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    windsock
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  3. 20th June 2010, 10:47 AM #3
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    80km/h
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  4. 20th June 2010, 10:57 AM #4
    clean32
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    (60 + 120 ) /2 = 90

    unless there is something else
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  5. 20th June 2010, 10:59 AM #5
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    80 km/hr
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  6. 20th June 2010, 11:08 AM #6
    RoverP6B
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    80km/h or 90km/h. Two answers but only one of them is correct.

    Which one is it? Who would like to change their mind? Who can prove that their answer is the correct one?

    Ron.
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  7. 20th June 2010, 11:12 AM #7
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    vnx205
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    Those who misread or misunderstand the question will say 60+120/2=90

    Those who read it carefully and realise it doesn't say "travel for half the TIME" rather than "half the DISTANCE" will say 2*60*120/(60+120)=80

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  8. 20th June 2010, 11:19 AM #8
    RoverP6B
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    That is exactly right vnx205...

    Equally well done to those who also worked out that 80km/h was the correct answer.

    Ron.
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  9. 20th June 2010, 11:20 AM #9
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    The easiest way I can think of to explain it is with an example.

    Imagine the distance to be covered is 120 km.

    The first 60 km at 60 km/h will take 1 hour.

    The second 60 km at 120 km/h will take half an hour.

    The whole trip took one and a half hours.

    Speed is distance divided by time.

    So 120 km divided by 1.5 hours = 80 km/h

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  10. 20th June 2010, 11:21 AM #10
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    Yep, the travel is referenced to distance, not to time.

    Sub in a distance eg 100km for the first half, and 100km for the 2nd half.

    The first 100km takes 1.66hrs to travel
    The 2nd 100km takes 0.83 hours to travel

    Total travel time for substituted 200km is 2.5 hours on average.
    Average over 100km is then 1.25hours which calculates out to 80km travel for the hour

    so 80km/h
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