If $\dfrac{a}{b} = \dfrac{c}{d} = \dfrac{e}{f}$, then $\dfrac{a+c+e}{b+d+f}$ equals—

Setting each ratio equal to $\dfrac{a}{b}$ and substituting $a=b\cdot\tfrac{a}{b}$, $c=d\cdot\tfrac{a}{b}$, $e=f\cdot\tfrac{a}{b}$, the numerator factors as $\tfrac{a}{b}(b+d+f)$. Dividing by $(b+d+f)$ leaves $\dfrac{a+c+e}{b+d+f}=\dfrac{a}{b}$.