The set of life spans of an appliance is normally distributed with a mean mu is equal to 48 months and a standard deviation sigma is equal to 8 months. What is the z - score of an appliance that stopped working at 64 months?

Use the z-score formula: z = (value − mean) ÷ standard deviation. For 64 months, z = (64 − 48) ÷ 8 = 16 ÷ 8 = 2, so the appliance’s life span is 2 standard deviations above the mean. The correct choice is 2.

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