An alloy contains tin and copper in the ratio 5:3. A second alloy contains tin and copper in the ratio 3:1. What quantity of the first alloy must be mixed with 20 kg of the second alloy to produce a new alloy with tin and copper in the ratio 4:2 (or 2:1)?

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?