Get x value with given probability
Your z score had the wrong sign. Since it wants the minimum burn time achieved by 90% of the lamps you are looking for the 10th percentile, not 90th percentile. So -1.25 standard deviations below the mean.
Get x value with given probability
Your z score had the wrong sign. Since it wants the minimum burn time achieved by 90% of the lamps you are looking for the 10th percentile, not 90th percentile. So -1.25 standard deviations below the mean.