Chemical Kinetics

For a reaction having three steps, the overall rate constant is $k = \dfrac{k_1 k_2}{k_3}$. The values of $E_{a_1}, E_{a_2}$ and $E_{a_3}$ (activation energies stepwise) are $40$, $50$ and $60$ kJ mol$^{-1}$ respectively. Then the overall $E_a$ (activation energy) of the reaction is ____