A machine has an efficiency of 80%. How much work must be done on the machine so to make it do 50,000 J of output work?

Efficiency is output work divided by input work. So 0.80 = 50,000 J ÷ input work, which gives input work = 50,000 J ÷ 0.80 = 62,500 J. The correct choice is 62,500 J.

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