Solució: \begin{align} &\cssId{Step1}{c_r= \frac{E_{útil}}{E_{cons}}} \\[3px] &\cssId{Step2}{E_{útil}=P_1·t_1} \\[3px] &\cssId{Step3}{c_r= \frac{P_1·t_1}{E_{cons}}} \\[3px] \end{align}