Efficiency from useful energy output

Steps:

• Calculate input energy: power × time = 6.0 W × 3.0 s = 18 J.
• Determine useful energy: total input - wasted = 18 J - 2.7 J = 15.3 J.
• Compute efficiency: useful energy / total input energy = 15.3 / 18 = 0.85.

Why D is correct:

• Efficiency is the ratio of useful output energy to total input energy, yielding 0.85 here.

Why the others are wrong:

• A: Wasted energy fraction (2.7/18 ≈ 0.15), not efficiency.
• B: Half the input energy (9/18 = 0.50, close but ignores exact waste).
• C: Approximate useful fraction but miscalculates (15.3/18 ≠ 0.55).

Final answer: D