Solution: Power,P=V2R=V2ρlAP'=1l-20100lP'P=ll-l/5=54P'-PP×100=54×100P'-PP×100=25%

How to do it

Solution:

P o w e r, P = \frac{V^{2}}{R} = \frac{V^{2}}{\frac{ρ l}{A}} P' = \frac{1}{l - \frac{20}{100} l} \frac{P'}{P} = \frac{l}{l - l / 5} = \frac{5}{4} \frac{P' - P}{P} \times 100 = \frac{5}{4} \times 100 \frac{P' - P}{P} \times 100 = 25 %