Efficiency as useful energy output over total input

Steps:

• Total energy input to kettle: 600 kJ.
• Total energy losses: 45 kJ (to surroundings) + 15 kJ (to casing) = 60 kJ.
• Useful energy to heat water: 600 kJ - 60 kJ = 540 kJ.
• Efficiency: (540 kJ / 600 kJ) = 0.90.

Why B is correct:

• Matches efficiency formula: useful output divided by total input equals 0.90.

Why the others are wrong:

• A: 0.10 inverts the ratio, treating losses as useful energy.
• C: 0.925 subtracts only surroundings loss (555 kJ / 600 kJ), ignoring casing.
• D: 0.975 subtracts only casing loss (585 kJ / 600 kJ), ignoring surroundings.

Final answer: B