Problem 2:
The lifespan of a particular brand of light bulb follows a normal distribution with a mean of 1000 hours and a standard deviation of 50 hours.
Find:
a) the z-score of light bulb with a mean of 500 hours.
b) If a customer buys 20 of these light bulbs, what is the probability that the average lifespan of these bulbs will be less than 980 hours?
c) the probability of light bulbs with the mean of 400 hours.
d) the number of light bulbs with the mean less than 1000 hours
