The life expectancy of Timely Brand watches is normally dist

The life expectancy of Timely Brand watches is normally distributed with a mean of four years and a standard deviation of eight months. The company has a three-year warranty period on their watches. What percentage of their watche will be in operating condition after the warranty period?

Solution

SD = 8 month

1 year = 12 month

SD year = 8 month

SD = 0.67

Z = 3 - 4   / 0.67

Z = -1.50

P( z > -1.50) = 1- 0.0668 = 0.9332