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Math Help - help me anyone

  1. #1
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    Cool help me anyone

    hi]


    two pipes a and b can seperately fill a tank in 20 mins and 30 mins respectively, which a third pipe C can empty the full tank iin 15 mins

    If all pipes are opened together then in what time is the empty tank filled?


    bye
    gautam93
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by gautam93 View Post
    hi]


    two pipes a and b can seperately fill a tank in 20 mins and 30 mins respectively, which a third pipe C can empty the full tank iin 15 mins

    If all pipes are opened together then in what time is the empty tank filled?


    bye
    gautam93
    The first pipe fills the tank at a rate of 1/20 of a tank/min
    The second pipe fills the tank at a rate of 1/30 of a tank/min

    The third pipe empties the tank at a rate of 1/15 of a tank/min.

    Starting from empty the content of the tank at time t is:

    t(1/20+1/30-1/15).

    The solution is the time t for which:

    t(1/20+1/30-1/15)=1,

    which is 60 minutes.

    RonL
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  3. #3
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    Here is a useful rule I develpoed that seems to help people.

    If it takes 'a' units of time to do a job and 'b' units of time to do the same job by two different things or people then it takes (ab)/(a+b) to do it together.
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by ThePerfectHacker View Post
    Here is a useful rule I develpoed that seems to help people.

    If it takes 'a' units of time to do a job and 'b' units of time to do the same job by two different things or people then it takes (ab)/(a+b) to do it together.
    Your rule does not lead to understanding, while the reason for it does.

    Though it seems to be the fashion of the day, I would discourage
    learning rules at the expense of understanding. I normally work many
    things, others seem to use rules for, from more primitive principles.

    RonL
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