Calculate the area of the shaded portion. Take C/A = 22/7.

Step 1: The shaded region is a triangle with base $20\,\mathrm{cm}$ and perpendicular height $15\,\mathrm{cm}$.

The formula for the area of a triangle is
$A = \frac{1}{2} \times base \times height.$

Step 2: Substitute the given values into the formula.
$A = \frac{1}{2} \times 20\,cm \times 15\,cm.$

Step 3: Calculate the product of base and height.
$20\,cm \times 15\,cm = 300\,cm^2.$

Step 4: Multiply by $\frac{1}{2}$.
$A = \frac{1}{2} \times 300\,cm^2 = 150\,cm^2.$

150 \mathrm{cm^2}