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Nice Easy One for the Weekend

Posted by Karl Sharman on January 28, 2011 – 1:10 pm

A car travels at a speed of 30 mph from A to B, a distance “x” , and then return from B to A over the same distance “x” at a speed of 20 mph. What is the average speed for the total trip?

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  1. 1. mike Said:

    In a a graph where
    X = time in units (t)
    Y = speed in units (mph)

    y = 30 when x = 0 to t
    y = 20 when x = t to 1.5t

    Take the area under the graph:
    30mph * t = 30t
    20mph * 1.5t = 30t

    add them

    30t + 30t = 60t

    Average over x axis
    60t/2.5t = 24mph

  2. 2. DP Said:

    24 MPH.
    time = dist / speed = x/30 + x/20 = x/12
    speed = dist / time = 2x/(x/12) = 24

  3. 3. Rohan Said:

    24 mph.

  4. 4. brett Said:

    25 mile per hour

  5. 5. brett Said:

    25 mile per hour
    Its really quite simple
    add the two speeds and divide by 2
    This will give you the average speed

  6. 6. Chris Said:

    Hi brett. You cannot simply take the average of the two speeds.

    For concreteness, let A and B be 60 miles apart. The outward journey, at 30 mph, woulf take 2 hours. The return journey, at 20 mph, would take 3 hours.

    The average speed for the round trip is the total distamce travelled divided by the total time taken = 120/5 = 24 mph.

  7. 7. Karl Sharman Said:

    Thank you Mike for the swift and correct answer

  8. 8. Serik Said:

    24 mph

  9. 9. mike Said:

    hah, woot. Totally did it graphically though : P. I got bugged trying to do it a different way.

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