The lifetimes of batteries produced by the juicy battery company are normally distributed with a mean of 100 hours and a standard deviation of 10 hours. find the number of hours where at least 90% of the batteries will continue to operate at.
The z-score associated with 90% is: P(0.9)=1.28 sup[pose the number of hours whereby at least 90% of the batteries will continue to operate is x. the z-score is given by: z=(x-μ)/σ where: μ is the mean σ is the standard deviation substituting the values we shall have: 1.28=(x-100)/10 solving for x we get: x=112.8 hours
