1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time and work

  1. Mar 4, 2010 #1
    1. The problem statement, all variables and given/known data
    The problem is as follows
    " 2 pipes fill cistern with water in 4 and 6 hours respectively.If the first pipe in 4 and 6 hours
    respectively . if the first pipe be opened first and the pipes be opened alternatively one at a time for 1 hour each, in how many hours will the cistern be filled up"
    My attempt:
    I first took that in 1 hour first pipe fills 1/4th of tank and second pipe 1/6 thof tank
    .: in 2 hours they fill 1/4+1/6= 5/12th of tank
    in 1hour they fill 5/24 th of tank
    .: they fill the tank in 24/5 hours
    But the problem is my book's answer is 19 hours.I don't know how that is possible because if they alone fill it in 4 and 6 hours then if they are working together the time should be less.
    Please help me!
  2. jcsd
  3. Mar 4, 2010 #2
    1/4 + 1/6 is = to 5/12 of the tank filled which is if both of them are filling the tank in one hour so you have to get a further 1/4 and 1/6 of these repectively so 1/4*5/12 and 1/6*5/12 and add them to get the time, maybe you'll get the rite answer...

    Hope this helps, confusing problem have to admit.
  4. Mar 5, 2010 #3
    I am really sorry:cry:, but could you please be more precise?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook