In a Wheatstone bridge (see figure) Resistance $P$ and $Q$ are approximately equal. When $R = 400\,\Omega ,$ the bridge is balanced. On interchanging $P$ and $Q,$ the value of $R,$ for balance is $405\,\Omega .$ The value of $X$ is close to ................. $ohm$
JEE MAIN 2019, Medium
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$\frac{P}{Q}=\frac{400}{s}$      .....$(1)$

and $\quad \frac{Q}{P}=\frac{405}{5}$      .......$(2)$

Solving $S^{2}=400 \times 405$

$\Rightarrow \quad S=402.5\, \Omega$

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