The true voltage drop across the three resistors in series is divided in proportion their resistances.
Voltage across 3000 ohm resistor V,
![](https://www.sarthaks.com/?qa=blob&qa_blobid=10497143418507796852)
When the voltmeter of resistance 6000 ohm is connected across 3000 ohm, the effective resistance of the 3000 ohm arm will be,
![](https://www.sarthaks.com/?qa=blob&qa_blobid=9713772070536952717)
So the voltmeter reading will be:
![](https://www.sarthaks.com/?qa=blob&qa_blobid=9310121407009495099)
Percentage error
![](https://www.sarthaks.com/?qa=blob&qa_blobid=12775710367369283113)
Voltmeter reading will be 16.6 % lesser than the true voltage across 3000 ohm resistor.