A voltmeter has a resistance of G ohm and range V volt. The value of resistance used in series to convert it into voltmeter or range nV volt is

We know that

$R=\frac{V}{I_g}-G$

The voltmeter gives the full-scale deflection for potential difference V. Its resistance is G. Hence, $I_g=(V/G).$Given that   $V-nV.$ Therefore, 

$R=\frac{nV}{(V/G)}-G=(n-1)G$