Explanation

(b) Let the resistance of voltmeter is $G\Omega.$
∴ Total resistance of the circuit
$R = \left( \frac{G \times 100}{G + 100} + 50 \right)\Omega$
Total current
$i = \frac{V}{G} = \frac{10}{\left( \frac{G \times 100}{G + 100} + 50 \right)}$
Voltage across 100$\Omega$ resistance
$= i\left( \frac{G \times 100}{G + 100} \right) = \frac{10}{\left( \frac{G \times 100}{G + 100} + 50 \right)} \times \left( \frac{G \times 100}{G + 100} \right)$
Reading of voltmeter =5V
∴ Voltage across 100$\Omega$ =5V
$\therefore 5 = \frac{10}{\left( \frac{G \times 100}{G + 100} + 50 \right)} \times \left( \frac{G \times 100}{G + 100} \right)$
On solving $G = 100\Omega$