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A voltmeter with an internal resistance of 200 Ω when connected across an unknown resistance reads 250 V. The milliammeter internal resistance = 0 connected in series with the above combination reads 10 mA. The actual value of the unknown resistance is ___________  
1. 25 kΩ 
2. 200 kΩ
3. 28.56 kΩ
4. 20 kΩ

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Best answer
Correct Answer - Option 3 : 28.56 kΩ

Voltmeter reading = 250 V

Ammeter reading = 10 mA

Hence, effective value of resistance

\({R_{eff}} = \frac{{250\;V}}{{10\;mA}} = 25\;k{\rm{\Omega }}\) 

Let us say that the value unknown resistance is R (the unknown resistance and the internal resistance will be in parallel) then, 

Reff = R 200 kΩ

⇒ \(25\;k{\rm{\Omega }} = \frac{{R \;\times\; 200}}{{R\; + \;200}}\)

R = 28.56 kΩ 

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