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Math Help - Word problem right?

  1. #1
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    Post Word problem right?

    As always I get stuck in word problems

    A local bus travels 7mph slower than the express. The Express travels 90miles in the same time it takes the local bus 75 miles find the speed of each bus.

    Here is my chart

    D R T
    local bus 75 7-X T
    EXPRESS 90 x? T

    x(x-7) 75/x-7 + x(x-7) 90/x = x(x-7)

    75x +90 x -630 = x^2 -7x

    and im lost... Did I even set up my problem correctly? thanks
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by rj2001 View Post
    As always I get stuck in word problems

    A local bus travels 7mph slower than the express. The Express travels 90miles in the same time it takes the local bus 75 miles find the speed of each bus.

    Here is my chart

    D R T
    local bus 75 7-X T
    EXPRESS 90 x? T

    x(x-7) 75/x-7 + x(x-7) 90/x = x(x-7)

    75x +90 x -630 = x^2 -7x

    and im lost... Did I even set up my problem correctly? thanks
    Ummmm...I'm not sure what you're doing with the "charts" so I'll show you how I would set it up.

    Call the speed of the express v. Then the speed of the bus is v - 7 (mph).
    Call the time it takes the express to travel 90 miles t. Then it also takes the bus a time t to travel 75 miles. Since d = vt:
    90 mi = vt (Express)
    75 mi = (v - 7)t (Bus)
    (I'll drop the units until the end for convenience.)

    So from the first equation:
    t = 90/v
    Insert this into the second equation:
    75 = (v - 7)*(90/v) <-- Multiply both sides by v:

    75v = 90v - 630

    15v = 630

    v = 42 mph
    v - 7 = 35 mph.

    -Dan
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  3. #3
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    lol thanks. My middle school teacher says its always a good idea to make a chart but it confuses me in which box to put what and i screw setting it up.
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  4. #4
    Senior Member DivideBy0's Avatar
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    Yes, setting up a Table Of Values for Distance, Velocity, and Time is very helpful, because you can simply pluck values from it when you need them to form equations. It also helps to draw a diagram of the problem, to help you visualize it, it is what my math teacher does with all Distance-Velocity-Time, type questions.
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