What is the voltage output of a transformer with a 500-turn primary and a 1500-turn secondary when 120 VAC is applied to the primary?
Why Transformer voltage follows the turns ratio: Vsecondary = Vprimary x (Nsecondary / Nprimary). Here the ratio is 1500/500 = 3, so 120 V x 3 = 360 volts. More turns on the secondary than the primary means a step-up transformer, so the output voltage must be higher than the input.
Watch out The 40 volt choice is what you get by dividing 120 by 3, which would be correct only if the winding with 1500 turns were the primary (a step-down connection).
More turns out = more volts out. Multiply by secondary/primary: 120 x (1500/500) = 360.