From a container, 6 liters milk was drawn out and was replaced by water. Again 6 liters of mixture was drawn out and was replaced by the water. Thus the quantity of milk and water in the container after these two operations is 8 : 10. The quantity of mixture is:

Let initial milk = x liters. After first replacement: milk = x(1 - 6/x). After second: milk = x(1 - 6/x)² = 8/18 = 4/9 of mixture. So (1 - 6/x)² = 4/9, meaning 1 - 6/x = 2/3 (positive root since 6/x < 1). Thus 6/x = 1/3, so x = 18 liters. Final milk:water ratio is 8:10 which simplifies to 4:5, confirming the calculation.