Answer
a) ${\bf 12,500}\;\rm turn$
b) ${\bf 2}\;\rm A$
Work Step by Step
$$\color{blue}{\bf [a]}$$
We know for transformers, which have two coils (primary coil+secondary coil), that
$$V_2=\dfrac{N_2V_1}{N_1}$$
where $_1$ refers to the primary coil, and $_2$ refers to the secondary coil.
Solving for $N_1$,
$$N_1=\dfrac{N_2V_1}{V_2}$$
Plug the known;
$$N_1=\dfrac{(100)(15,000)}{(120)}=\color{red}{\bf 12,500}\;\rm turn$$
$$\color{blue}{\bf [b]}$$
Since we know that $P_{\rm in}=P_{\rm out}$ where $P_{\rm out}$ is from the secondary coil, and $P_{\rm in}$ is from the primary coil,
$$I_1V_1=I_2V_2$$
Solving for $I_1$ to find the current in the primary coil.
$$I_1=\dfrac{I_2V_2}{V_1}$$
Plug the known;
$$I_1=\dfrac{(250)(120)}{(15,000)}=\color{red}{\bf 2}\;\rm A$$
