Area growth of a pond ripple after ten seconds

A circular ripple spreads out on a pond, its radius growing at $2$ ft per second from zero. After $10$ seconds the ripple's area is increasing at $k\pi$ square feet per second for an integer $k$. What is $k$?

$k=80$. After $10$ s the radius is $20$ ft. From $A=\pi r^2$, $A'=2\pi rr'=2\pi\cdot20\cdot2=80\pi$.