A certain type of lightbulb is advertised to have an average lifetime of 1,000 hours. Assume the lifetimes of these lightbulbs are approximately normally distributed with a standard deviation of 250 hours.

Question

A certain type of lightbulb is advertised to have an average lifetime of 1,000 hours.  Assume the lifetimes of these lightbulbs are approximately normally distributed with a standard deviation of 250 hours.
A) Find the percentage of lightbulbs that will fail during their first 250 hours of use 
B) Find the percentage of lightbulbs that will fail within 100 hours of their average lifetime (either 50 hours more or less than average).
C) At what number of hours should the warranty lifetime be set so that fewer than 2% of the bulbs must be replaced under warranty?
 
Not sure what formulas to use in excel to calculate these answers.  Norm.dist or Norm.Inv

PsyDAG · Answer

Z = (score-mean)/SD

A,B. Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability related to the Z scores. Multiply by 100.

C. Use same table to find Z for .02. Insert in above equation to find score.