The effective wattage of 40 W and 60 W lamps connected in series with each other is
(A) 160W (B) 80W (C) 40W (D) 24 W 

Dear Student,
Answer to your question is given below:
Given,  40 W and 60 W lamps connected in series with each other
To find the power of their combination.
let P₁ and P₂ are the powers of two given bulbs A and B respectively and both are connected in series combination.
resistance of bulb A, R₁ =$\frac{V^2}{P_1}$
resistance of bulb B, R₂ =$\frac{V^2}{P_2}$
We know, equivalent resistance of two resistors which are joined in series combination is given by,
        R_eq = R₁ + R₂
$\Rightarrow \frac{V^2}{P_{eq}} = \frac{V^2}{P_1} +\frac{V^2}{P_2}$
$\Rightarrow \frac{1}{P_{eq}} = \frac{1}{P_1} + \frac{1}{P_2}$ , where P_eq is power of their series combination.
Now using above formula to find power of combination
Here P₁ = 60 W,              P₂ = 40 W
so, $\frac{1}{P_{eq}} = \frac{1}{60}+ \frac{1}{40}$
$$\begin{gathered} \Rightarrow \frac{1}{P_{eq}}=\frac{40+60}{60 \times 40 } \\ \Rightarrow \frac{1}{P_{eq}}=\frac{100}{2400} \\ \Rightarrow P_{eq}=24 W \end{gathered}$$

so, P_eq = 24 W
Therefore the power of their combination is 24W. Hence, the correct choice is option (D)
Regards,