17. The lifetime of a certain type of battery is normally distributed with mean value 10 hours and standard deviation 1 hour. There are four batteries in a pack- age. What lifetime value is such that the total life- time of all batteries in a package exceeds that value for only 5% of all packages?
18. Rockwell hardness of pins of a certain type is known to have a mean value of 50 and a standard deviation of 1.2.
a. If the distribution is normal, what is the proba- bility that the sample mean hardness for a ran- dom sample of 9 pins is at least 51?
b. What is the (approximate) probability that the sample mean hardness for a random sample of 40 pins is at least 51?
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