The manufacturer of an extended-life lightbulb claims the bulb has an average life of 12,000 hours, with a standard deviation of 500 hours. If the distribution is bell shaped and symmetrical, what is the approximate percentage of these bulbs that will last
a. between 11,000 and 13,000 hours?
b. over 12,500 hours?
c. less than 11,000 hours?
d. between 11,500 and 13,000 hours?