Evaluate Device Energy Efficiency Practice Test

A heat engine takes in thermal power $P_{in}=900,\text{W}$ from burning fuel and produces useful mechanical power $P_{out}=270,\text{W}$. The rest is waste heat to the surroundings. What is the engine’s efficiency (as a percentage)?

Two light bulbs produce light (useful) and heat (waste).

• Incandescent bulb: $P_{in}=100\ \text{W}$ electrical, $P_{out}=5\ \text{W}$ light.
• LED bulb: $P_{in}=20\ \text{W}$ electrical, $P_{out}=8\ \text{W}$ light.

Which bulb is more efficient, and by how many percentage points (LED efficiency minus incandescent efficiency)?