Matrices & Systems · Medium problem

A furniture shop makes chairs and tables. Each chair uses $3$ board-feet of wood and $2$ pounds of metal fittings; each table uses $5$ board-feet of wood and $4$ pounds of metal fittings. This week the shop makes $40$ chairs and $25$ tables. Writing the per-unit needs as $R = \begin{pmatrix}3 & 5\\2 & 4\end{pmatrix}$ (rows: wood, metal; columns: chair, table) and the production numbers as $\mathbf p = \begin{pmatrix}40\\25\end{pmatrix}$, how many board-feet of wood does the shop need this week?