An accelerated life test on a large number of typeD alkaline

An accelerated life test on a large number of type-D alkaline batteries revealed that the mean life for a particular use before they failed is 21.0 hours. The distribution of the lives approximated a normal distribution. The standard deviation of the distribution was 1.4 hours. About 95 percent of the batteries failed between what two values?

Solution

Mean = 21.0 and std dev =1.4

Z value for 95% is 1.96

Hence lower value = 21-1.4(1.96) = 18.256

Upper value = 23.744

95% of the batteries failed between 18.256 and 23.744.