Question

Two lamps rated 60w and 240v are connected in series. What is the total power dissipated in both.

Answer 1

The resistance of one lamp

$$R_1=\frac{V^2}{P_1}=\frac{(240\:\rm{V})^2}{60\:\rm{W}}=960\:\Omega$$

The resistance of two lamps connected in series

$$R=2R_1=2\times 960\:\Omega=1920\:\Omega$$

Finally, the total power dissipated in both lamps is

$$P=\frac{V^2}{R}=\frac{(240\:\rm{V})^2}{1920\:\Omega}=30\:\rm{W}$$