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Math Help - Pipes and cistern..need your answer.

  1. #1
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    Pipes and cistern..need your answer.

    Two pipes A and B can fill a tank in 8 hours. If only pipe A is open, then it would take 4 hours to fill the tank. Find how much longer would it take if only pipe B is open.

    My answer is 24 hours but the book answer is 16, kindly help.
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  2. #2
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    Re: Pipes and cistern..need your answer.

    Quote Originally Posted by hisajesh View Post
    Two pipes A and B can fill a tank in 8 hours. If only pipe A is open, then it would take 4 hours to fill the tank. Find how much longer would it take if only pipe B is open.

    My answer is 24 hours but the book answer is 16, kindly help.
    something doesn't make sense here ... both pipes fill the tank in 8 hrs, but pipe A (by itself) fills the tank in 4 hrs ???
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  3. #3
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    Re: Pipes and cistern..need your answer.

    Sorry typo...it is 4 hours longer*
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    Re: Pipes and cistern..need your answer.

    In other words pipes A and B together fill this in 8 hours and B alone fills it in 12 hours.

    Whenever you have a problem like this, whether it is pipes or cars or whatever, it it the speeds that add. Let x be the time it would take for A alone to fill the cistern. The speed with which it fills is 1/x. Let y be the time it takes B alone to fill the cistern: y= 12 hours. The speed with which it fills is 1/y= 1/12 "cistern per hour". The two together fill the cistern in 8 hours so 1/x+ 1/12= 1/8.
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  5. #5
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    Re: Pipes and cistern..need your answer.

    rate of A = (1 tank)/(12 hrs)

    rate of B = (1 tank)/(t hrs)

    8(rate of A + rate of B) = 1 tank filled

    I agree with your solution ...

    8[(1/12) + (1/24)] = 1
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