Answer
$\begin{bmatrix}28&39&15\\30&28.5&13\end{bmatrix}$
Work Step by Step
In part b) we built the matrix $M+J$:
$$\begin{align*}
M+J&=\begin{bmatrix}56&78&30\\60&57&26\end{bmatrix}.
\end{align*}$$
The matrix $(1/2)(M+J)$ represents the average sales for the two month period for each store and each model. Determine the matrix $(1/2)(M+J)$:
$$\begin{align*}
\dfrac{1}{2}(M+J)&=\dfrac{1}{2}\begin{bmatrix}56&78&30\\60&57&26\end{bmatrix}\\
&=\begin{bmatrix}28&39&15\\30&28.5&13\end{bmatrix}.
\end{align*}$$
