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