In a 160-litres mixture of milk and water, the percentage of water is only 35%. The milkman gave 40 litres of this mixture to a customer and then added 'X' litres of pure milk to the remaining mixture. As a result the percentage of water in the final mixture becomes 30%. What is the value of 'X'?

Initial mixture: 160L with 35% water = 56L water, 104L milk. After removing 40L, remaining water = 56 × (120/160) = 42L, milk = 104 × (120/160) = 78L. After adding X liters milk, total mixture = 120 + X liters with 42L water. Given water becomes 30%: 42/(120 + X) = 30/100, so 4200 = 3600 + 30X, therefore 30X = 600, X = 20 liters.