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Gnagey's Gknives claim they will last at least year 10 years without needing to be sharpened.

Consumer Reports randomly samples 42 knives, and calculates that they need to be sharpened on average in 9.25 years. Based on other knife sharpening studies, suppose that the population standard deviation is known to be 1.7 years. Consumer Reports wants to conduct this analysis at 0.01 level of significance.

What is the sample size?