In a mixture of Milk and water some mixture is taken out & water is added in place of mixture hence percentage of water increased to 40% from 20%. If 12 litre of water was added . Find the quantity of milk in mixture in starting?

When mixture is removed, milk and water are removed proportionally to their concentrations. Let initial milk = M. After removing x litres and adding 12L water, milk = 0.8(M-x), total = (M-x)+12. New water% = 40% means milk% = 60%, so 0.8(M-x) / (M-x+12) = 0.6. Solving gives M-x = 36, so milk = 0.8 × 36 = 28.8L. Key insight: milk quantity doesn't change when water is added.