In a Wheatstone bridge (see fig.),

In a Wheatstone bridge (see fig.), Resistances $P$ and $Q$ are approximately equal. When $\mathrm{R}=400 \Omega$, the bridge is balanced. On interchanging $P$ and $Q$, the value of $R$, for balance, is $405 \Omega$. The value of $\mathrm{X}$ is close to: