The local authorities in a certain city instal 10,000 electric lamps in the
streets of the city. If these lamps have an average life of 1,000 burning hours
with a standard deviation of 200 hours, assuming normality, what number of
lamps might be expected to fail i). in the first 800 burning hours?
ii). between 800 to 1200 burning hours?
After what period of burning hours would you expect that
(a) 10% of the lamps would fail?
(b) 10% of the lamp will be still burning?