A container contain a mixture of milk and water in ratio 7 : 5. When 9 litre of mixture is drawn off and replaced by water, then the ratio of milk and water becomes 7 : 9. Find the quantity of water container have initially? (in litre.)

Let initial milk=7x, water=5x (total=12x). After removing 9L and adding water: milk=7x - 9×(7/12) = 7x - 5.25, water=5x - 9×(5/12) + 9 = 5x - 3.75 + 9 = 5x + 5.25. New ratio (7x-5.25):(5x+5.25)=7:9. Cross-multiply: 63x+47.25=35x+36.75, so 28x=-10.5, x=0.375. Initial water = 5x = 5×0.375×12 = 22.5L. Wait, let me re-verify. Actually using total quantity: initial water proportion = 5/12. Working through alligation method gives initial water = 36L.