## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$R=\frac{er}{E-e}$
Use the rule $\frac{a}{b}=\frac{c}{d} \longrightarrow ad=bc$ to obtain: $$E\cdot R=e(R+r) \\ER=eR+er$$ Subtract $eR$ from both sides: $$ER-eR = er$$ Factor out $R$: $$R(E-e)=er$$ Divide $E-e$ to both sides: $$\frac{R(E-e)}{E-e}=\frac{er}{E-e} \\R=\frac{er}{E-e}$$