# Question #4dd1e

Oct 22, 2016

$20 W$

#### Explanation:

Let the resistance of each 60W bulb be $R \Omega$
When connected in series the combination will have equivalent resistance $= 3 R \Omega$

Power of each lamp $P = 60 W$

If supply voltage be $E \text{ Volt}$ then $\text{Power} \left(P\right) = {E}^{2} / R$
$\implies 60 = {E}^{2} / R$

When the combination is connected to the mains, then total power consumed by the combination will be

${P}_{\text{comb}} = {E}^{2} / \left(3 R\right) = \frac{1}{3} \times 60 W = 20 W$