A $100\, V$ voltmeter of internal resistance $20\,k\Omega $ in series with a high resistance $R$ is connected to a $110\, V$ line. The voltmeter reads $5\, V$, the value of $R$ is ................ $k \Omega $
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(c) Here $i = \frac{{110}}{{20 \times {{10}^3} + R}}$
$V = iR$
$ \Rightarrow $ $5 = \left( {\frac{{110}}{{20 \times {{10}^3} + R}}} \right) \times 20 \times {10^3}$
$ \Rightarrow $ ${10^5} + 5R = 22 \times {10^5}$
$ \Rightarrow $ $R = 21 \times \frac{{{{10}^5}}}{5} = 420\,K\Omega $
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