Two electric bulbs rated 25 W, 220 V and 100 W, 220 V are connected in series across a 220 V voltage source. The 25W and 100 W bulbs now draw P₁ and P₂ powers, respectively.

Let V = 200 V and R₁ and R₂ be the resistances of the 25 W and 100 W bulbs.

${\mathrm{P}}_1=25={\mathrm{V}}^2/{\mathrm{R}}_1\text{ or }{\mathrm{R}}_1={\mathrm{V}}^2/25\text{ and }{\mathrm{R}}_2={\mathrm{V}}^2/100$

When the bulbs are joined in series, the current is

$\mathrm{I}=\frac{\mathrm{V}}{{\mathrm{R}}_1+{\mathrm{R}}_2}$

Power in the 25 w bulb is ${\mathrm{R}}_1{\mathrm{I}}^2$ and in the 100 w bulb is ${\mathrm{R}}_2{\mathrm{I}}^2$.