A filament bulb (500 W, 100 V) is to be used in a 230 V main supply. When a resistance R is connected in series, it works perfectly and the bulb consumes 500 W. The value of R is.

Using P=VI the current through bulb is I=P/Vb = 500/100 = 5A and the resistance of the bulb is Rb=V2b/P = 100²/500 = 20Ω
As the resistance R is connected with bulb in series so same current will flow through them, i.e I=5A
By Ohm's law, 230 = (R+Rb)I or 230 = (R+20)(5) or R = 26Ω