When 45 liter water is mixed in a mixture containing 60\% milk, concentration of milk in mixture becomes 45\%. Find the initial quantity of mixture.

Let initial mixture be x liters containing 60% milk, so milk = 0.6x. After adding 45 L water, total = x + 45 and milk concentration becomes 45%, so 0.6x / (x + 45) = 0.45. Solving: 0.6x = 0.45(x + 45) = 0.45x + 20.25, so 0.15x = 20.25, x = 135. Verification: Initial milk = 0.6 × 135 = 81 L. After adding water: 81/180 = 45%.