Final answer:
The useful energy of the small fan is 10 joules.
Step-by-step explanation:
The useful energy of the small fan is 10 joules. The useful energy is the amount of energy that is converted into a desired form of energy by a device, in this case, the small fan. The other options, 30 joules, 45 joules, and 5 joules, are not the correct amount of useful energy for the small fan.
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