There are two containers $X$ and $Y$ . $X$ contains 100 ml of milk and $Y$ contains 100 ml of water. 20 ml of milk from $X$ is transferred to $Y$ . After mixing well, 20 ml of the mixture in $Y$ is transferred back to $X$ . If $m$ denotes the proportion of milk in $X$ and $n$ denotes the proportion of water in $Y$ , then which one of the following is correct?