At $100°\text{C}$, the gaseous reaction $\text{A} \rightarrow 3\text{B} + \text{C}$ is found to be first order. Starting with pure A, at the end of $20$ minutes the total pressure of the system is $190\,\text{mm}$ and after a long time, when dissociation of A was complete, it was $400\,\text{mm}$. The partial pressure of A at the end of $20\,\text{min}$ is: