Answer
$0.36\times\pi\times t^{2}$
Work Step by Step
The notation can be rewritten as $A(r(t))$.
This can be found by substituting the function, r(t) into A(r).
$A(r)=\pi \times r^{2}$ , $r(t)=0.6t$,
Therefore $A(r(t))=\pi \times (r(t))^{2}$
$=\pi \times (0.6\times t)^{2}$
=$0.36\times\pi\times t^{2}$
The meaning of this expression is the rate of change of the area of circle per second, $t$
