In a certain quantity of water, four times the quantity of milk is added, making the total mixture 7.5 liters. How much more milk must be added to this mixture so that the mixture contains 90% milk?

A vessel has 20 litres of milk. From this vessel, 5 litres of milk is taken out and replaced by water. The process is repeated once more. Find the quantity of milk (in litres) remaining in the vessel.

A shopkeeper mixes 40 kg of rice costing ₹30/kg with 60 kg of rice costing ₹50/kg. What is the average price (in ₹/kg) of the mixture?