The ideal transformer (either step-up or step-down) works with the equation:

\(\frac{V_{s}}{V_{p}} = \frac{N_{s}}{N_{p}} = \frac{I_{p}}{I_{s}}\)

where \(V_{s}\) and \(V_{p}\) are the voltages applied to the secondary and the primary respectively

\(N_{s}\) and \(N_{p}\) are the number of turns on the secondary and primary coils respectively

\(I_{p}\) and \(I_{s}\) are the currents flowing in the primary and secondary coils respectively.

\(\frac{V_{s}}{100} = \frac{200}{100}\)

\(V_{s} = \frac{200\times 100}{100} = 200V\)