We have a flash drum separating $50$ kmol$/$h of a mixture of ethane, isobutane, and n$-$butane. The ratio of isobutane to n$-$butane is held constant at $0\mathrm{.}8($that is$,$ ziC$4/$znC$4=0\mathrm{.}8).$ The mole fractions ofall three components in the feed can change. The flash drum operates at a pressure of $1000$ kPa and a temperature of $20\backslash$deg C$.$ If the drum is operating at V$/$F $=0\mathrm{.}4,$ what must the mole fractions of all three components in the feed be$?$