UKMT Grey Kangaroo, 2019

Some identical rectangles are drawn on the floor. A triangle of base $10$ cm and height $6$ 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) $10 \text{ cm}^2$ (B) $12 \text{ cm}^2$ (C) $14 \text{ cm}^2$ (D) $15 \text{ cm}^2$ (E) $21 \text{ cm}^2$