Step 1: Convert into Matrices

Quantity Matrix (in dozens):

\[ Q = \begin{bmatrix} 12 & 5 & 6 \\ 10 & 6 & 7 \\ 11 & 13 & 8 \end{bmatrix} \]

Cost per item (in ₹):

\[ \text{Notebook} = 0.40,\quad \text{Pen} = 1.25,\quad \text{Pencil} = 0.35 \] \[ C = \begin{bmatrix} 0.40 \\ 1.25 \\ 0.35 \end{bmatrix} \]

Step 2: Multiply (Cost of 1 dozen = 12 items)

Multiply by 12: \[ C = \begin{bmatrix} 4.8 \\ 15 \\ 4.2 \end{bmatrix} \]

Step 3: Find Total Cost

\[ \text{Total} = Q \times C \] \[ = \begin{bmatrix} 12 & 5 & 6 \\ 10 & 6 & 7 \\ 11 & 13 & 8 \end{bmatrix} \begin{bmatrix} 4.8 \\ 15 \\ 4.2 \end{bmatrix} \]

Step 4: Calculate

\[ = \begin{bmatrix} (12×4.8 + 5×15 + 6×4.2) \\ (10×4.8 + 6×15 + 7×4.2) \\ (11×4.8 + 13×15 + 8×4.2) \end{bmatrix} \] \[ = \begin{bmatrix} 157.2 \\ 167.4 \\ 281.4 \end{bmatrix} \]

Final Answer