An $80\Omega$  galvanometer deflects full-scale for a potential of 20  mV. A voltmeter deflecting full-scale of 5 V is to be made using this galvanometer. We must connect

The current through the galvanometer producing full-scale deflection is

$I=\frac{V}{R}=\frac{20\times {10}^{-3}}{80}=2.5\times {10}^{-4}A$

To convert the galvanometer into a voltmeter, a high resistance connected in series with the galvanometer. Therefore, 

$5V=(2.5\times {10}^{-4})(R+80)orR=19.92k\Omega$