A trader mixes 6ltr of milk costing 5000 rupees with 7ltr of milk costing 6000 rupees per litre. The trader also mixes some quantity of water to the mixture so as to bring the price to 4800 per litre. How many litres of water is added?

Question: A trader mixes 6ltr of milk costing 5000 rupees with 7ltr of milk costing 6000 rupees per litre. The trader also mixes some quantity of water to the mixture so as to bring the price to 4800 per litre. How many litres of water is added?

Solution:
(6 × 5000 + 7 × 6000)/(13 + w) = 4800
⇒ 30000 + 42000 = 62400 + 4800w
⇒ 4800w = 72000 - 62400
⇒ w = 9600/4800
∴ w = 2

w is the amount of water added, w = 2 litre