#### Answer

(a)$$r=2t$$
(b)$$(V \circ r)(t)= \frac{32}{3} \pi t^3$$ This composite function represents the volume of the inflating spherical balloon in terms of time.

#### Work Step by Step

(a) The value of the radius of the inflating spherical balloon equals the velocity times the elapsed time. So we have $$r=2t.$$
(b)$$V=\frac{4}{3} \pi r^3$$ $$\Rightarrow \quad (V \circ r)(t)=V(r(t))=\frac{32}{3} \pi t^3$$