a tank can be filled by one pipe in 3.5 hours and emptied by another in 4.2 hours. if both pipes are running, how long will take to fill an empty tank ?
Net rate of fillup = 1/3.5 - 1/4.2 tanks/h = 0.04762 tanks/h
Time to fill = 1 tank/0.04762 tank/h
= 21 hours