Isotermik

\begin{equation*}
\begin{split}
\frac {P_1 \:.\: V_1}{\cancel {T_1}} & = \frac {P_2 \:.\: V_2}{\cancel {T_2}} \\\\
P_1 \:.\: V_1 & = P_2 \:.\: V_2 \\\\
400 \:.\: V_1 & = P_2 \:.\: 2 \:.\: V_1 \\\\
P_2 & = \frac {400 \:.\: \cancel {V_1}}{2 \:.\: \cancel {V_1}} \\\\
P_2 & = 200 \text{ Pa}
\end{split}
\end{equation*}