A gas mixture of CH$4$ and He is contained in a tube at $25$ deg C and $14\mathrm{.}7$ psia. At one point the partial pressure of CH$4$ is $60\mathrm{.}79$ kPa and at a point $2$ cm distance away, the partial pressure of CH$4$ is $20\mathrm{.}26$ kPa. Calculate the flux of CH$4$ at steady$-$state for equimolar diffusion.