Step-by-step explanation:
It is given:
[tex]\mu=1000[/tex], [tex]\sigma=200[/tex]
Let x be the random variable that denotes the number of hours the lamp lights up.
what number of lights might expected to fail in the first 800 burning hours
Answer: We are required to find the expected number of lights to fail in the first 800 burning hours.
First, we have to find the probability as:
P(Z < 800)
Using the z-score formula, we have:
Using the z-score formula, we have:
[tex]P(x <800)=P \left(z<\frac{800-1000}{200} \right )[/tex]
=P(z<-1)
Now using the standard normal table, we have:
P(x<800)=P(z<-1)=0.1587
Therefore, the expected number of lights to fail in the first 800 burning hours is:
[tex]\boldsymbol{0.1587 \times 10000=1587}[/tex]
After how many burning hours 10% of the lamps would be still burning?
We first need to find the z-value corresponding to area = 1- 0.10 = 0.90. Using the standard normal table, we have:
z(0.90)=1.28
Now using the z-score formula, we have:
[tex]z= \frac{x-\mu}{\sigma}[/tex]
[tex]1.28= \frac{x-1000}{200}[/tex]
[tex]1.28 \times 200= x-1000[/tex]
[tex]256= x-1000[/tex]
x= 1000+256
[tex]\boldsymbol{x= 1256 }[/tex]
Using the Zscore principle, the number of bulbs expected to Fail in the first 800 burning hours is 1587
Given the Parameters :
The number of Lamps that might be expected to fail in the first 800 burning hours :
Recall:
Zscore = (800 - 1000) / 200 = -1
Using the normal distribution table :
P(Z < -1) = 0.15866
The number of bulbs expected to fail :
0.15866 × 10000 = 1586.6
Hence, 1587 bulbs are expected to fail in the first 800 burning hours.
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