Posted by patel on .
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 ?

maths 
drwls,
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