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

Approx 992 bulbs will die after a useful life of 900hrs

Given:

The life of 1000 bulbs normally distributed

Mean life 1620hrs

Standard deviation 300hrs

To Find:

the number of bulbs which would die out after a useful life of 900hrs

Solution:

Mean = 1620 hrs

SD = 300 hrs

life = 900 hrs

Z score = ( x - Mean) / SD

=> Z score = ( 900 - 1620)/300

=> Z score = - 2.4

for z=2.4 area is 0.4918

Hence Total area  = 0.5 + 0.4918  = 0.9918

Number of bulbs = 1000 * 0.9918  = 991.8 ≈ 992

Approx 992 bulbs will die after a useful life of 900hrs

None of the given option are correct

1000 - 992 = 8  Bulbs will die before a useful life of 900hrs