Question 1172199
z for the 10th percentile of failure is -1.282
length of life is z=(x-mean)/sd
so -1.282*60=x-360
-76.92=x-360
x=283 days
-
Practicality is the amount of work needed--replacing may be time intensive. The cost of replacing all bulbs has to be balanced against the issues should one burn out. Where is it?
-
B bulbs that lasts 450 days, 90 days longer, will require all to be replaced (same policy) in 373 days.
-
If B lasts 25% longer, by the means, it is exactly 25% longer. But because the sd is less per the mean, the 10th percentile for B is 31% longer than it is for A costing 25% more. It's a close call slightly in favor of B.
-
Type C would be -1.282*45=x-432
so x=374.31.  It is slightly more than B, and has less variability, generally a good thing, Its costs should be similar to B although slightly less would be reasonable(0.3%).