The mathematical representation of a machine's efficiency is given by the formula:
\[ \text{Efficiency} = \frac{E_o}{E_i} \times 100 \]
Where:
- \( E_o \) is the useful output energy (or work done),
- \( E_i \) is the input energy (or work put in).
Therefore, the correct response is:
Efficiency = \( \frac{E_o}{E_i} \times 100 \)