A tank contains 10 liters of a 15% alcohol solution. To obtain 10 liters of a 70% solution, approximately how many liters of this solution must be replaced with an 80% alcohol solution?

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?