If a transformer has a primary with 100 V and fifty coils, and a secondary that yields 20 V, how many coils are on the secondary? a.) 200 b.) 20 c.) 50 d.) 10 e.) 100

Answer :

Ideally, if all the magnetic of one winding cuts the other winding, and there isn't any loss in the transformer core or the resistance of the wire, then the voltage across each winding is proportional to the number of turns in its coil.

If you apply 100 V to a winding of 50 turns, then a winding that yields 20 volts
must be wound with

(20/100) of 50 turns = 10 turns