In what ratio must water be mixed with milk costing Rs. 15 per litre to obtain a mixture worth Rs. 10 per litre? (Assume water is available free of cost)

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?