A 40 L mixture of petrol and kerosene is mixed with 60 L of another mixture that contains petrol and kerosene in the ratio 7: 3. If the final mixture contains petrol and kerosene in the ratio 13: 7, what is the difference between the quantities of petrol and kerosene in the first mixture?

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?