The correct equation for a machine's efficiency is:

Efficiency = $\frac{\text{output energy}}{\text{input energy}} \times 100$

This is because efficiency is defined as the ratio of useful output energy to input energy, expressed as a percentage.

The correct option from the image is:

$\text{Efficiency} = \frac{\text{output energy}}{\text{input energy}} \cdot 100$.

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Related Questions:

1. What is the significance of efficiency in mechanical systems?
2. How does energy loss affect the efficiency of a machine?
3. Can a machine's efficiency exceed 100%? Why or why not?
4. What are some common methods to increase the efficiency of a system?
5. How does efficiency differ between an ideal and a real-world machine?

Tip:

Always ensure to measure input and output energies in the same units (e.g., joules) to calculate efficiency accurately.