A flashlight battery manufacturer makes a model of battery whose mean shelf life is three years and four months, with a standard
deviation of three months. The distribution is approximately normal. One production run of batteries in the factory was 25,000 batteries. How many of those batteries can be expected to last between three years and one month and three years and seven months?
Three years and one month is equivalent to the mean minus one standard deviation.
Three years and seven months is equivalent to the mean plus one standard deviation.
For a normal distribution, we know that 68% of population is between mean ± 1 sd, then can be expected that 25000*68% = 17000 of batteries last between three years and one month and three years and seven months
