Efficiency Formula for Power Systems

Steps:

• Identify total power input as sum of useful output and wasted power: 4 W + 16 W = 20 W.
• Apply efficiency formula: (useful power / total power) × 100%.
• Calculate: (4 W / 20 W) × 100% = 20%.
• Confirm result matches choice B.

Why B is correct:

• Efficiency is defined as η = (P_useful / P_total) × 100%, where P_total = P_useful + P_wasted, directly yielding 20%.

Why the others are wrong:

• A: 5% incorrectly uses wasted power fraction (16/20 × 25% miscalculation).
• C: 25% wrongly divides useful by wasted only (4/16 × 100%).
• D: 80% inverts efficiency, using wasted fraction (16/20 × 100%).

Final answer: B