Lightbulbs have a normally distributed lifetime of 5000 hours with a standard deviation of 2000 hours. There are two rooms and four new lightbulbs in each room are turned on

(a) What is the expected length of time until one of the rooms is completely dark?

(b) What is the probability that the length of time until one of the rooms is completely dark is more than 7000 hours?

(c) What is the probability both rooms are dark at 6000 hours or less?

Use an Excel simulation with 1000 iterations to answer these questions.