Question

A city installs 2000 electric lamps for street lighting. These lamps have a mean burning life of 1000 hours with a standard deviation of 200 hours. The normal distribution is a close approximation to this case.

i)       What is the probability that a lamp will fail in the first 700 burning hours?

ii)      What is the probability that a lamp will fail between 900 and 1300 burning hours?

iii)     What is the probability that a lamp will burn for exactly 900 hours?

iv)     What is the probability that a lamp will fail after 1100 burning hours?

v)      After how many burning hours would we expect 10% of the lamps to be left?

Answer 1