I have this question: “An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be: (a) 100 W (b) 75 W (c) 50 W (d) 25 W”

I use P=VI to find Current when Power is 100 W and Voltage is 220 V, then I find Current = 5/11 A. And then I use P=VI to find Power when Voltage is 110 V and Current is 5/11 A. I find that the Power is 50 W.

But when I use P=V²/R to find Resistance when Power is 100 W and Voltage is 220 V, then I find Resistance to be 484 Ω. Then I use P=V²/R to find Power when Voltage is 110 V and Resistance is 484 Ω. I find that the Power is 25 W.

Our teacher has used the later method. Why can't we use the first one and why is it wrong?​