Efficiency from useful energy over total input

Steps:

• Convert time: 30 min = 1800 s.
• Total energy input: 15 W × 1800 s = 27,000 J.
• Useful energy: 27,000 J - 7,000 J wasted = 20,000 J.
• Efficiency: 20,000 J / 27,000 J = 0.74.

Why D is correct:

• Matches efficiency formula: (useful energy / total energy input) = (total - wasted) / total.

Why the others are wrong:

• A: Assumes total energy as 27,000 J but subtracts incorrectly, yielding ~0.26.
• B: Divides wasted by useful (7,000 / 20,000 = 0.35), inverting the formula.
• C: Uses time in minutes (15 × 30 = 450, then misapplies wasted), giving ~0.59.

Final answer: D