Answer
$r=\dfrac{A-P}{Pt}$
Work Step by Step
Distribute $P$ to obtain:
$A=P+Prt$
Subtract $P$ on both sides to obtain:
$A-P=Prt$
Divide $Pt$ on both sides to obtain:
$\dfrac{A-P}{Pt} = \dfrac{Prt}{Pt}$
Cancel the common factors to obtain:
$\require{cancel}
\dfrac{A-P}{Pt} = \dfrac{\cancel{P}r\cancel{t}}{\cancel{Pt}}
\\\dfrac{A-P}{Pt} = r$
