Question:
A bulb is made using two filaments. A switch selects whether the filaments are used individually or in parallel. When used with a $15 \mathrm{~V}$ battery, the bulb can be operated at $5 \mathrm{~W}, 10 \mathrm{~W}$ or $15 \mathrm{~W}$. What should be the resistances of the filaments?
Solution:
