Everything depands on the middle piece, so we have to write the length of the board in terms of the middle piece.
[tex]\begin{gathered} 4M\text{ =L} \\ M-5=S \\ L+M+S=67 \\ \therefore\rightarrow \\ 4M+M+(M-5)=67in \\ 4M+M+M=72in\rightarrow6M=72in\rightarrow M=\frac{72}{6}in=12in \end{gathered}[/tex]Therefore the longest piece is
[tex]L=4M=4\cdot(12)=48in[/tex]