15. Some identical rectangles are drawn on the floor. A triangle of base $10 \mathrm{~cm}$ and height $6 \mathrm{~cm}$ is drawn over them, as shown, and the region inside the rectangles and outside the triangle is shaded. What is the area of the shaded region?
A $10 \mathrm{~cm}^{2}$
B $12 \mathrm{~cm}^{2}$
C $14 \mathrm{~cm}^{2}$
D $15 \mathrm{~cm}^{2}$
E $21 \mathrm{~cm}^{2}$
15. B Since the length of five identical rectangles is $10 \mathrm{~cm}$, the length of one rectangle is $2 \mathrm{~cm}$. Similarly, since the height of four rectangles is $6 \mathrm{~cm}$, the height of one rectangle is $1.5 \mathrm{~cm}$. Therefore the total area of the 14 rectangles is $14 \times(2 \times 1.5) \mathrm{cm}^{2}=42 \mathrm{~cm}^{2}$. Hence the area of the shaded region is equal to $\left(42-\frac{1}{2} \times 10 \times 6\right) \mathrm{cm}^{2}=(42-30) \mathrm{cm}^{2}=12 \mathrm{~cm}^{2}$.