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Thread: Uniform Motion

  1. #1
    Member harpazo's Avatar
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    Uniform Motion

    A motorboat heads upstream on a river that has a current of 3 mph. The trip upstream takes 5 hours and the return trip takes 2.5 hours. What is the speed of the motorboat? (Assume that the motorboat maintains a constant speed relative to the water.)

    Distance is not given here.

    I assume that r•t = r•t is what we need to set up the equation to find the speed of the motorboat.

    Upstream: (r - 3); time = 5 hours

    Downstream: (r + 3); time = 2.5 hours

    Equation:

    (r - 3)5 = (r + 3)(2.5)

    Correct?
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  2. #2
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    Re: Uniform Motion

    Yes, that is correct.
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  3. #3
    Member harpazo's Avatar
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    Re: Uniform Motion

    (r - 3)5 = (r + 3)(2.5)

    5r - 15 = 2.5r + 7.5

    5r - 2.5r = 7.5 + 15

    2.5r = 22.50

    r = 22.50/2.5

    r = 9

    The speed of the motorboat is 9 mph.
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  4. #4
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    Re: Uniform Motion

    CHECK your answer YOURSELF:

    9 - 3 = 6: 6*5 = 30
    9 + 3 =12: 12*2.5 = 30

    So correct...
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  5. #5
    Member harpazo's Avatar
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    Re: Uniform Motion

    I check the answer myself on paper.
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