Question

A transformer changes the 10,000 v power line to 120 v. if the primary coil contains 750 turns, how many turns are on the secondary?

Answer 1

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:
$\frac{N_p}{N_s}= \frac{V_p}{V_s}$
Where $N_p$ and $N_s$ are the number of turns in the primary and secondary coils, while $V_p$ and $V_s$ are the voltages on the two coils.

Using the data of the problem: $N_p=750$, $V_p=10000 V$ and $V_s=120 V$, we can find $N_s$, the number of turns of the secondary coil:
$N_s=N_p \frac{V_s}{V_p}=750 \frac{120 V}{10000 V}=9$