The isothermal gas phase reaction is $A+2B4R.$ Consider a feed $C_{A0}=100$ moles $\frac{?}{l},C_{B0}=200$ moles $\frac{?}{l}$ to a steady$-$state flow reactor operating at constant pressure. If $C_B=50$ moles $\frac{?}{l}$ at the reactor exit. What are $x_A$ and $x_B?$