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:
[tex] \frac{N_p}{N_s}= \frac{V_p}{V_s} [/tex]
Where [tex]N_p[/tex] and [tex]N_s[/tex] are the number of turns in the primary and secondary coils, while [tex]V_p[/tex] and [tex]V_s[/tex] are the voltages on the two coils.
Using the data of the problem: [tex]N_p=750[/tex], [tex]V_p=10000 V[/tex] and [tex]V_s=120 V[/tex], we can find [tex]N_s[/tex], the number of turns of the secondary coil:
[tex]N_s=N_p \frac{V_s}{V_p}=750 \frac{120 V}{10000 V}=9 [/tex]
