To measure a potential difference across a resistor of resistance R W a voltmeter of resistance R_v is used.  To measure the potential with a minimum accuracy of 95 % then 

$R_v \ge 19 R$

$ \text{Let Initial current is I}$

$\text{After voltmeter was inserted the current through resistor R is } I'= I \frac{R_v}{R+R_v} \ge 0.95I$

$ \frac{R_v}{R+R_v} \ge 0.95 $

$ R_v \ge 0.95 R + 0.95 R_v$

$ \Rightarrow 0.05 R_v \ge 0.95R $

$ \Rightarrow R_v \ge 19R$