Question

I don’t know how to start this

Answer 1

Look at the picture.
The area of the shaded area is equal the area of the circle subtract from half the area of the rectangle.
$A_1=\dfrac{1}{2}\cdot20\cdot10=100\\\\r=20:4=5\\\\A_2=\pi 5^2=25\pi$

$A_F=A_1-A_2\to A_F=100-25\pi$