pls show how to use the McWilliams equation

Problem 3. A given mixture is 35 mole\% n-pentane (C5) with unknown amounts of isobutane (iC4) and n-heptane (C7). Experiments using this mixture in a flash drum resulted in yic4=0.65 at 50C and 200kPa. Find the mole fraction of iso-butane in the feed, zic4, to two decimal places and the value of V/F to three decimal places. Use the McWilliams equation. An energy balance is not required to solve the