Three hundred joules of heat are added to a system. The system uses 140 joules to do work.

By the first law of thermodynamics, the change in the system’s internal energy is the heat added minus the work done by the system: ΔU = Q − W. Substituting the values gives ΔU = 300 J − 140 J = 160 J, so the correct choice is 160 J.

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