$2-$ Consider a binary distillation of n$-$butane and i$-$pentane. The equilibrium $x-y$

data for the light key $($n$-$butane$)$ at distillation conditions is given on the following

page. The feed is an equilibrium mixture of equimolar amounts vapor and liquid

containing $50$mol$\%$ of each component. The distillate must be $90\%$ n$-$butane and

the bottoms must be $90\%$ i$-$pentane. Given a reflux ratio of $3$ times the minimum

necessary reflux ratio, how many stages $($including the reboiler$)$ are necessary to

accomplish the separation? Which stage is the feed stage?