How is the mechanical efficiency of a machine usually expressed 1. as a fraction 2. as a percentage 3. as inches or feet 4. as one of three classes

The efficiency of any contraption that uses energy to
do a job is defined as

         (amount of useful energy output) / (amount of energy input) .

In describing or talking about the efficiency f the machine,
it's most often expressed as a percentage.

It's important to notice that the output is taken to be the useful output.
Why is that important ? 
Here's a simple example:

If you use an incandescent light bulb as a source of light, then
the useful output is the light, and its efficiency is quite low.

If you use an incandescent light bulb as a source of heat, then
the useful output is the heat, and its efficiency is quite high.

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