An electric bulb, marked 40 W and 200 V is used in a circuit of supply voltage 100 V. Now its power is

Power of bulb is given by, $\mathrm{P}=\frac{{\mathrm{V}}^2}{\mathrm{R}}$

$\begin{array}{l}\Rightarrow \frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}=\frac{{\mathrm{V}}_2^2}{{\mathrm{V}}_1^2}\left(\because \mathrm{R}\mathrm{is}\mathrm{constant}\right)\\ \Rightarrow \frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}={\left(\frac{100}{200}\right)}^2=\frac{1}{4}\Rightarrow {\mathrm{P}}_2=\frac{{\mathrm{P}}_1}{4}=\frac{40}{4}=10\mathrm{W}\end{array}$