For an exothermic reaction $\mathrm{A}\to \mathrm{B}$ the activation energy is $65\mathrm{kJ}{\mathrm{mol}}^{-1}$ and enthalpy of reaction is$42\mathrm{kJ}{\mathrm{mol}}^{-1}$ .The activation energy for the reaction $\mathrm{B} \to \mathrm{A}$ would be