Answer
The area of each subrectangle is $\Delta A = 1$.
The number of subrectangles is $32$.
Work Step by Step
The Riemann sum ${S_{8,4}}$ implies that $N=8$ and $M=4$. The domain is a rectangle, where $1 \le x \le 5$ and $2 \le y \le 10$.
Since the partition is regular, we have
$\Delta x = \frac{{5 - 1}}{8} = \frac{1}{2}$, ${\ \ \ \ }$ $\Delta y = \frac{{10 - 2}}{4} = 2$
So, the area of each subrectangle is $\Delta A = \Delta x\cdot\Delta y = 1$. The number of subrectangles is $N\cdot M = 32$.
