The ratio of voltage in the Primary Coil (V_p) is to the induced voltage in the Secondary Coil (V_s) equals to the ratio of the number of loops in Primary Coil (N_p) to that in the number of loops in the Secondary Coil (N_s). Thus,
             V_p               N_{p
               ---------  =  ------------}
                   V_s              N_s

So, if you have an appliance at home that operates on 110 volts AC but your wall outlet is a 220 Volts AC source, you should use a transformer. If the number of loops in the primary coil is 2000, how many loops should the secondary coil have?

From the equation we have:

               N_p V_s
 N_{s   =     --------------}
                         V_p

               (2000)(110 V)

 N_{s   =     --------------------------}
                      220 V

       =          1000 loops