12,000 coils. The primary winding has 12,000 coils.
In order to solve this problem we have to use the relation for a transformer Vp/Vs = Np/Ns, where Vp is the voltage across the primary winding, Vs in the voltage across the secondary winding, Np is the numbers of coils in the primary winding, and Ns is the number of coils en the secondary winding.
With Vp = 350,000 V, Vs = 17,500 V, and Ns = 600 coils:
350,000 V/17,500 = Np / 600 coils
Np = (600 coils x 350,000 V) / 17,500 V
Np = 210,000,000/ 17,500 coils
Np = 12,000 coils
