The lifespan of a particular brand of light bulb is normally distributed with a mean life of 400 h and a standard deviation of 12 h. Production for the last week was 800,000 light bulbs. Use this information to answer the following questions.
a) What is the z value of a light bulb having a life of 421 h?
b) How many light bulbs can be expected to have a life less than 379 h?
c) What percent of the light bulbs should have a life less than 430 h?