Assume a voltage source supplies an AC voltage of amplitude V(o) between its output terminals. If the output terminals are connected to an external circuit, and an ac current of amplitude I(o) flows out of the terminals then the equivalent resistance of the external circuit is R(eq) = V(o) / I(o).
If a transformer with N(p) and N(s) turns in its primary and secondary, respectively, is placed between the source and the resistor (primary facing voltage source, secondary facing resistor), what is R(eq)?