#### Answer

(a)$$r=60t$$
(b)$$A= 3600 \pi t^2$$
This function represents the area of growing ripple in terms of time.

#### Work Step by Step

(a) The value of the increasing radius equals the velocity times the elapsed time. So we have $$r=v \times t=60t.$$
(b) $$A= \pi r^2 \quad \Rightarrow$$ $$(A \circ r)(t)=A(r(t))=3600 \pi t^2$$