A Carnot engine has a source at 600 K and a sink at 400 K. Which change gives the higher efficiency, and by how much: raising the source temperature by 50 K, or lowering the sink temperature by 50 K?

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Raising the source: η 1 = 1 - 400/650 = 0.3846. Lowering the sink: η 2 = 1 - 350/600 = 0.4167. η 2 - η 1 = 0.0321, i.e. about 3.2 percentage points in favour of lowering the sink. (Both beat the initial 33.3%; the 8.3-point gain in (d) is the gain of η 2 over the initial efficiency, not over η 1 .) Matches (b).

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