## College Algebra (6th Edition)

$r=\frac{S-P}{Pt}$
$S=P+Prt$ Solve for $r$ Add $-P$ to both sides. $S-P=P+Prt-P$ $S-P=Prt$ Divide by $Pt$ both sides $\frac{S-P}{Pt}=\frac{Prt}{Pt}$ $\frac{S-P}{Pt}=r$ $r=\frac{S-P}{Pt}$