There are three pipes connecting a large water tank to a pool. Each of the pipes fills the pool at a uniform rate but the rates of different pipes are different. To fill the empty pool, one can open the first pipe for 2 hours and then the second pipe for 1 hour. Also using the second pipe for 2 hours and the third pipe for 1 hour fills the empty pool. If the pool is empty and the first pipe was open for 80 minutes, then the second pipe was open for 80 minutes, for how long does the third pipe need to be open after that to fill the pool?
Plz help quickly - this is due 2moro!!!