In a transformer, let's say we have:
I1, I2 - currents through the primary and secondary winding
V1, V2 - voltages
N1, N2 - number of turns
F1, F2 - magnetic fluxes through core, produced by the currents I1 and I2 (they are opposing...)
R - the reluctance of the core
We have V1/V2 = N1/N2...