A machine takes in 500 J and delivers 400 J of useful output. Its efficiency is

A. 80 per cent
B. 125 per cent
C. 100 per cent
D. 20 per cent

Explanation

Dividing 400 by 500 and multiplying by 100 gives 80 per cent, with the remaining 100 J lost mainly as heat. Dividing the wrong way round gives 125 per cent, which should be recognised at once as impossible. The 20 per cent figure is the fraction wasted rather than the efficiency.

A machine takes in 500 J and delivers 400 J of useful output. Its efficiency is

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