How many liters of water should be added to a 30-liter mixture of milk and water containing milk and water in the ratio of 7 : 3 such that the resultant mixture has 40% water in it?

The total quantity of mixture = 30 liters
Quantity Milk in the mixture = 7/(7 + 3) × 30 litres = 21 litres
and quality water in the mixture = 3/ (7 + 3) × 30 litre = 9 litre
Let water to be mixed 'a' litre
Then (30 + a) × 40/100 = 9 + a
or, 120 + 4a = 90 + 10a
or, 120 - 90 = 10a - 4a
30 = 6a
⇒ a = 5
Hence, 5 litres water mixed in the mixture.