Carlota does 2000 J of work on a machine. The machine does 500 J of work. What is the efficiency of the machine?

Efficiency is useful output work divided by input work, multiplied by 100%. The machine outputs 500 J from 2000 J of input, so its efficiency is (500 ÷ 2000) × 100% = 25%. Therefore, the correct choice is 25%.

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