In a chemistry laboratory, there are two varieties of an acid. Variety A costs Rs. 80 per litre, while variety B costs Rs. 100 per litre. In what proportion must a scientist mix both varieties A and B so that the mixture becomes worth Rs. 92 per litre?

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?