If R₁ and R₂ are the resistances of filaments of a 400 W and a 200 W lamp, designed to operate on the same voltage, then:

If a wire of resistance R is melted and recast to half of its length, then new resistance of the wire will be:

n equal resistors are first connected in series and then connected in parallel. The ratio of the maximum to the minimum resistance is:

In a circuit containing two unequal resistors connected in parallel:

The equivalent resistance in series combination is: