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Math Help - Word probelm help!!!

  1. #1
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    Word probelm help!!!

    I have a review due tomorrow and these word problems are killing me!!! I would greatly appreciate it if anyone could help, here they are

    1. The force of a wind varies jointly with the window surface area and the square of the velocity of the wind. If the force is 40 pounds on a surface area of 12 square feet when the wind is 10 mph, what is the force on a surface area of 20 square feet when the wind is 30 mph?

    2. How many liters of water must be added to 20 liters of a 5% salt solution to dilute it to a 1% salt solution?

    3. A boat can travel 20 mph in still water. It can travel 75 miles downstream in the same time that it requires to travel 45 miles upstream. Find the rate of the stream.

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  2. #2
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    e^(i*pi)'s Avatar
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    Quote Originally Posted by Help! View Post
    I have a review due tomorrow and these word problems are killing me!!! I would greatly appreciate it if anyone could help, here they are

    1. The force of a wind varies jointly with the window surface area and the square of the velocity of the wind. If the force is 40 pounds on a surface area of 12 square feet when the wind is 10 mph, what is the force on a surface area of 20 square feet when the wind is 30 mph?

    2. How many liters of water must be added to 20 liters of a 5% salt solution to dilute it to a 1% salt solution?

    3. A boat can travel 20 mph in still water. It can travel 75 miles downstream in the same time that it requires to travel 45 miles upstream. Find the rate of the stream.
    1. Let the window surface be A (ft)^2 and the wind speed be v mph

    F \propto Av^2

    <br />
40 = k \times 12 \times 10^2

    k  = \frac{40}{1200} = \frac{1}{30}

    F_2 = kAv_2^2 = \frac{1}{30} (20)(30^2) = 600 Apparently your units should be pounds but that's not a unit of force...

    ---------------

    2. Assuming you mean percentage by volume:

    A 5% solution in 20L means that there is 20*0.05L = 1L of salt. To make this a 1% solution you need to keep the 1 = 0.01*V. V = 100 and so you need to add 100-20 (for you have 20 already) = 80L

    (you may want to get this checked)

    ------------

    3.Let the current be v mph.

    Going downriver you have \frac{75}{20+v}

    Going upriver you have \frac{45}{20-v}

    Equate these to find v: \frac{75}{20+v} = \frac{45}{20-v}

    75(20-v) = 45(20+v)

    1500 - 75v = 900+45v

    600 = 120v

     v = 5mph
    Last edited by e^(i*pi); April 15th 2009 at 02:47 PM.
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  3. #3
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    thank you very much!!!
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