The following distribution shows the daily pocket allowance of children of a locality. The mean pocket allowance is Rs 18. Find the missing frequency \( f \). \( \begin{array}{|c|c|} \hline \text{Daily pocket allowance (in ₹)} & \text{Number of children} \\ \hline 11 - 13 & 7 \\ \hline 13 - 15 & 6 \\ \hline 15 - 17 & 9 \\ \hline 17 - 19 & 13 \\ \hline 19 - 21 & f \\ \hline 21 - 23 & 5 \\ \hline 23 - 25 & 4 \\ \hline \end{array} \)