Answer
140 ohms
Work Step by Step
First find the resistance of each bulb from the power and the voltage applied across it.
$$P=\frac{V^2}{R}$$
$$R_{75W}=\frac{V^2}{P}=\frac{(120\;V)^2}{75W}=192\Omega$$
$$R_{25W}=\frac{V^2}{P}=\frac{(120\;V)^2}{25W}=576\Omega$$
When resistors are wired in parallel, we use equation 19–4 to find the equivalent resistance.
$$\frac{1}{R_{eq}}=\frac{1}{192\Omega}+\frac{1}{576\Omega}$$
$$R_{eq}=144\Omega\approx140\Omega$$
