Answer
$\dfrac{32 \pi}{81}t^9; t \geq 0$
Work Step by Step
We have: $V(r)=\dfrac{4}{3} \pi r^3$ and $r(t)=\dfrac{2}{3}t^3$.
Therefore, the composite function will be:
$V(r(t))=\dfrac{4}{3} \pi r^3 \\= \dfrac{4}{3} \pi (\dfrac{2}{3}t^3)^3=\dfrac{32 \pi}{81}t^9; t \geq 0$
