For a reaction taking place in four different steps, the rate constants follow a relationship \(K-\frac{K_2K_4}{K_1K_3}\). If the energies of activation E1, E2, E3 and E4 are 10 kJ/mol, 30kJ/mol, 40 kJ/mol and 20 kJ/mol respectively, then the overall energy of activation would be :