The table below shows information about the thickness of each of the books on a shelf.
Work out an estimate for the mean book thickness.
Give your answer as a decimal.
\begin{tabular}{|c|c|}
\hline Thickness, [tex][tex]$x \, ( \text{mm} )$[/tex][/tex] & Frequency \\
\hline [tex][tex]$0 \ \textless \ x \leq 2$[/tex][/tex] & 4 \\
\hline [tex][tex]$2 \ \textless \ x \leq 4$[/tex][/tex] & 9 \\
\hline [tex][tex]$4 \ \textless \ x \leq 6$[/tex][/tex] & 7 \\
\hline
\end{tabular}