Using P = RV2, we get R = PV2∴ resistance of bulb 1002202Ω
Power of bulb across changed voltage, P ' = 2202/100[220(0.8)]2 = 100(0.8)2 W
But in fact resistance of bulb does not remain constant due to change in temperature of the filament. Therefore, at reduced voltage the temperature of filament will be less i.e. resistance of filament will be less and giving power more than 100(0.8)2 W.