The ideal transformer (either step-up or step-down) works with the equation:
VsVp=NsNp=IpIs
where Vs and Vp are the voltages applied to the secondary and the primary respectively
Ns and Np are the number of turns on the secondary and primary coils respectively
Ip and Is are the currents flowing in the primary and secondary coils respectively.
Vs100=200100
Vs=200×100100=200V
