Let the initial amount of milk be 7 x litres and the initial amount of water be 5 x litres. After replacing 9 litres of milk with water: New amount of milk =7x−9 litres New amount of water =5x+9 litres New ratio of milk to water =(7x−9):(5x+9) Given that the new ratio is 7:9, we can set up the equation: