3 5000 electric lamps are installed in the city of Saskatoon

3. 5000 electric lamps are installed in the city of Saskatoon for street lighting. The burning lifetime of these lamps are normally distributed and have a mean burning life of 1400 hours with a standard deviation of 250 hours. a) What is the probability that a lamp will fail in the first 800 burning hours? b) What is the probability that a lamp will fail between 1200 and 1700 burning hours? c)How many lamps are expected to fail between 1200 and 1700 burning hours?

Solution

a)P=(1400-800)/250

=2.4

find the z-value of 2.4 in z-table

b)z1=(1400-1200)/250

=.8

Z2=(1400-1700)/250

     = -1.2

so probability will be

z1-z2 and then calculate the value from z-table