Consider one mole of helium gas enclosed in a container at initial pressure $P_1$ and volume $V_1$. It expands isothermally to volume $4 V_1$. After this, the gas expands adiabatically and its volume becomes $32 V_1$. The work done by the gas during isothermal and adiabatic expansion processes are $W_{\text {iso }}$ and $W_{\text {adia }}$, respectively. If the ratio $\frac{\mathrm{W}_{\text {iso }}}{\mathrm{W}_{\text {adia }}}=f \ln 2$, then $f$ is $\_\_\_\_$ .