The following table shows the expenditure of 60 boys on books. Find the mode of their expenditure:

    \[\begin{tabular}{|l|l|} \hline \text { Expenditure } & \text { No. of } \\ \text { (Rs) } & \text { students } \\ \hline \text { 20-25 } & 4 \\ \hline \text { 25-30 } & 7 \\ \hline 30-35 & 23 \\ \hline 35-40 & 18 \\ \hline 40-45 & 6 \\ \hline \text { 45-50 } & 2 \\ \hline \end{tabular}\]