A machine is a device which turns one type of energy into another. The efficiency of any device is defined as:

**Efficiency = Useful energy OUT / Total energy IN**

You might not know the energy inputs and outputs of a machine, but you can still calculate the machine's efficiency as long as you know the power input and output:

**Efficiency = Useful power OUT / Total power IN**

You can give efficiency as a decimal or you can multiply your answer by 100 to get a percentage, i.e. 0.75 or 75%.

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