A step-up transformer in a school laboratory has 200 turns in its primary and 3000 turns in its…

Step 1. The secondary voltage. $V_s = 120\ \text{V} \times \dfrac{3000}{200} = 1800$ V.

Step 2. The secondary current. $I_s = \dfrac{V_s}{R} = \dfrac{1800\ \text{V}}{9000\ \Omega} = 0.20$ A.

Step 3. The power. $P = V_sI_s = 1800\ \text{V} \times 0.20\ \text{A} = 360$ W.

Step 4. The primary current. We want $I_p$, so $I_p = \dfrac{P}{V_p} = \dfrac{360\ \text{W}}{120\ \text{V}} = 3.0$ A.