Question

A bulb is made using two filaments. A switch selects whether the filaments are used individually or in parallel. when used with a 15V battery, the bulb can be operated at $5W,10W$ or $15W$. What should be the resistances of the filaments?

Answer 1

The various resistances of the bulbs $=\frac{V^2}{P}$

Resistances are $\frac{(15{)}^2}{10},\frac{(15{)}^2}{10},\frac{(15{)}^2}{15}=45,22.5,15$.

since two resistance when used in parallel have resistance less than both.

The resistances are $45$ and $22.5$.