Answer
$ r=\dfrac{S-a}{S}$
Work Step by Step
Multiply both sides by $1-r$.
$S(1-r)=(1-r)\cdot \dfrac{a}{1-r}$
Simplify by cancelling common factors.
$S(1-r)=a$
Use distributive property.
$S-Sr=a$
Subtract $S$ from both sides.
$S-Sr-S=a-S$
Simplify.
$-Sr=a-S$
Divide both sides by $-S$.
$\dfrac{-Sr}{-S}=\dfrac{a-S}{-S}$
Simplify.
$r=\dfrac{S-a}{S}$
