The lifetime of a certain brand of lightbulbs is normally distributed with the mean of 3800

hours and standard deviation of 250 hours.

a. The probability that randomly selected lightbulb will have lifetime more than 3450 hours is

_________.

b. The percent of lightbulbs which have the lifetime between 3450 and 4250 hours is ___________.

c. What lifetime should the manufacturer advertise for these lightbulbs if he assumes that 10

% of lightbulbs with the smallest lifetimes will burn out by that time?

Advertised time is _________________ hours.