Answer
$r=\dfrac{A-P}{Pt}$
Work Step by Step
Using the properties of equality, then, in terms of $
r
,$ the given equation, $
A=P+Prt
,$ is equivalent to
\begin{array}{l}\require{cancel}
A-P=Prt
\\\\
\dfrac{A-P}{Pt}=r
\\\\
r=\dfrac{A-P}{Pt}
.\end{array}