A pot contains 81 litres of pure milk. \(\frac{1}{3}\) of the milk is replaced by the same amount of water. Again \(\frac{1}{3}\) of the mixture is replaced by that amount of water. What is the ratio of milk and water in the new mixture ?
1
1 : 2
2
2 : 5
3
3 : 7
4
4 : 5