Question

An oil slick is expanding as a circle. The radius of the circle is increasing at the rate of 20 meters per day. Express the area of the circle as a function of d, the number of days elapsed.

Answer 1

The circle expands it radius 20m each day. From this description you can get this equation:
r=0+ 20*d
r=20d

The oil slick expands as a circle and the formula for circle area is $\pi$ r^2.  If you insert the first equation into this equation you will got:
circle area = $\pi$ r^2
circle area = $\pi$ (20d)^2
circle area = $\pi$ 400*d^2