Voltage = 5 volts
[tex]\sf \dfrac{number \ of \ turns \ in \ primary \ coil}{voltage\ in \ primary \ coil} = \dfrac{number \ of \ turns \ in \ secondary\ coil}{voltage\ in \ secondary\ coil}[/tex]
[tex]\hookrightarrow \sf \dfrac{400}{100} = \dfrac{20}{V_2}[/tex]
[tex]\hookrightarrow \sf 400V_2}{} = 20*100[/tex]
[tex]\hookrightarrow \sf V_2 = 5 V[/tex]
