wo pipes x and y fill a tank in 15 hrs. and 20 hrs. respectively, while a third pipe ‘z’ can empty the full tank in 25 hrs.  all the three pipes are opened in the beginning.  after 10 hrs. z is closed.  in how much time, will the tank be full ?

wo pipes x and y fill a tank in 15 hrs. and 20 hrs. respectively, while a third pipe ‘z’ can empty the full tank in 25 hrs.  all the three pipes are opened in the beginning.  after 10 hrs. z is closed.  in how much time, will the tank be full ?