Answer
$R=\frac{Ir}{1-I}$
Work Step by Step
In order to solve the equation
$I=\frac{R}{R+r}$
for $R$, we must isolate $R$ on one side.
Multiply both sides by $R+r$ to clear denominators:
$I\cdot(R+r)=R$
$I\cdot R+I\cdot r=R$
Subtract $I\cdot R$ from each side and factor out $R$:
$I\cdot r=R-I\cdot R=R(1-I)$
Divide both sides by $1-I$:
$R=\frac{Ir}{1-I}$
