Final answer:
The efficiency of a system that expends 150 J of energy to produce 75 J of output is 50%, calculated using the formula: Efficiency (%) = (Useful Output / Total Input) × 100.
Step-by-step explanation:
The efficiency of an engine or system represents the ratio of the useful output of energy to the total input energy, expressed in percentage. In this case, to calculate the efficiency of a system where 150 J of energy are expended to produce 75 J of energy, you would use the formula:
Efficiency (%) = (Useful Energy Output / Total Energy Input) × 100
Plugging in the given values:
Efficiency = (75 J / 150 J) × 100 = 50%
Therefore, the correct answer is (a) 50%