Answer:
68% of all batteries will have a life between 32 months and 40 months.
Explanation:
The Empirical Rule states that, for a normally distributed random variable:
68% of the measures are within 1 standard deviation of the mean.
95% of the measures are within 2 standard deviation of the mean.
99.7% of the measures are within 3 standard deviations of the mean.
In this problem, we have that:
Mean = 36.
Standard deviation = 4.
68% within 1 standard deviation of the mean.
36 - 4 = 32
36 + 4 = 40
68% of all batteries will have a life between 32 months and 40 months.