Find the mode of the following classified data. \[ \begin{array}{|c|c|c|c|c|} \hline \text{Class interval} & 0\text{-}10 & 10\text{-}20 & 20\text{-}30 & 30\text{-}40 \\ \hline \text{Frequency} & 3 & 4 & 6 & 5 \\ \hline \end{array} \]