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1000 light bulbs with a mean life of 120 days are installed in a new factory
and their length of life is normally distributed with standard deviation of 20 days.
If it is decided to replace all the bulbs together, what interval should be allowed
between replacements if not more than 10% should expire before replacement?
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You are given a normal curve with the mean of 120 days and the standard deviation
of 20 days. They want you determine the raw mark t days such that the area under
this normal curve on the left from this mark is 10% of the total area under the curve.
For this purpose, you can use a standard function invNorm in your regular calculator
TI-83 or TI-84. invNorm stands for inverse cumulative normal distribution function.
mark mean SD <<<---=== formatting pattern
P = invNorm(0.1, 120, 20).
The ANSWER is 94 days (rounded to the closest integer day).
Alternatively, you can use this online calculator
https://onlinestatbook.com/2/calculators/inverse_normal_dist.html
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Solved.