A step-down transformer is 94 % efficient at full load…

Step 1. The power out. $P_{out} = V_sI_s = 24\ \text{V} \times 15\ \text{A} = 360$ W.

Step 2. The power in. We want $P_{in}$, so $P_{in} = \dfrac{P_{out}}{\eta} = \dfrac{360\ \text{W}}{0.94} = 383$ W.

Step 3. The primary current. $I_p = \dfrac{P_{in}}{V_p} = \dfrac{383\ \text{W}}{230\ \text{V}} = 1.67$ A.

Step 4. The power lost. $P_{in} - P_{out} = 383\ \text{W} - 360\ \text{W} = 23$ W.