An equimolar mixture of $A$ and $B$ (A being more volatile) is flash distilled continuously at a feed rate of $100 \text{ kmol/h}$, such that the liquid product contains $40\: mol\:\%$ of $A$. If the relative volatility is $6$, then the vapor product, in $\text{kmol/h}$, is

1. $10$
2. $20$
3. $25$
4. $45$