Let A represent the battery life of a brand A laptop, and let B represent the battery life of a brand B laptop. The mean difference in battery life between these two brands of laptops, D = A – B, is –3.25 hours. What is the correct interpretation of this value?

The difference is defined as D = A − B. Since the mean difference is −3.25 hours, brand A’s average battery life is 3.25 hours less than brand B’s. Equivalently, brand B’s battery life lasts 3.25 hours longer on average, so choice D is correct. This compares battery life between the brands; it does not describe daily gains or losses.

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