Question

Question 38
The life of 1000 bulbs normally distributed
with mean life 1620hrs and standarad
deviation 300hrs. Find the number of bulbs
which would die out after a useful life of
900hrs for z=2.4 area is 0.4918
990
10
90
8

Answer 1

Answer:

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Answer 2

Answer:

The number of bulbs  which would die out after a useful life of 900hrs is option A

Step-by-step explanation:

• The given question is solved using the standard normal distribution.
• It is a bell shaped disribution having symmetry about mean.
• The z value of the standard normal distribution is given by

      $z=\frac{x-u}{\sigma}$

      where x is the value for which probability is to be determined, u is

      the  mean and σ is the standard deviation.

Step 1 of 1:

• Mean is given by 1620
• The standard deviation is 300
• The value of z for 900 hrs is given by

       $z=\frac{x-u}{\sigma}$

       $z=\frac{900-1620}{300}\\z=-2.4$

• For z =-2.4, the probability is 0.0082
• for the 1000 bulbs, the number of bulbs that die after 900 hrs is

      $n=1000(1-.0082)\\n=991.8$

Hence the approx answer is option A