For a transformer, the ratio between the number of turns of primary and secondary coil is the same as the ratio between the voltages on the two coils:
Where
and 
are the number of turns in the primary and secondary coils, while 
and 
are the voltages on the two coils.
Using the data of the problem:
, 
and 
, we can find , the number of turns of the secondary coil:
Where
Using the data of the problem: