Question

an electric bulb is rated for 220 v and 100 watts . when it is operated on 110 volts , what is the power consumed ?

Answer 1

P = 110 Watts    V = 220 Volts RMS            P = V²/R
Resistance of bulb is R = V² / P = 220²/110  = 440 Ohms

Power consumed at V = 110 Volts RMS  :
   V² / R  = 110² / 440  = 27.5 Watts

Answer 2

p=v^2/R
 p=220x220/R
R=484
 now v=110 volts, R=484
p=v^2/R=110x110/484= 25watt