1. The problem statement, all variables and given/known data A step-down transformer has an efficiency of 90%, the voltage of it primary coil is 200V and its current is 0.5A, and its secondary coil has a voltage of 9V and a current of 10A and its number of turns is 90 turns. Calculate the number of turns of the primary coil. 2. Relevant equations η =VsIs/VpIp * 100 Vs/Vp = Ns/Np Is/Ip = Np/Ns 3. The attempt at a solution I solved the problem in two slightly different methods, the first one: And the second one: The solution in the first method (Np = 1800) is the correct one, but I don't understand why the second one is wrong. I thought that both will lead me to the same answer, but apparently they didn't, could you explain it for me, please?