"And pour the water in 60 litre container from bottom into 7 litre container!! Now remainig mixture is 30 litre oil and 17 litre water!!" I don't think you can make a hole or something on the bottom of the container! The question doesn't say that there is a tap, either. "Now as u have 7 litre container, u can easily separate 28 litre water from that mixture!! "---- How? Again, use the "hole" to pour the water into the container? If there is a tap on the bottom, then, your seperating water from the oil is convincing; but I think it has to be done assuming that there is no tap or a hole. I am going to give this a try. It may be only tomorrow or sometimg tonight, though. Lotsa assignments...