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Sagot :
Answer:
The mean life time of the bulbs is approximately 739 hours
Step-by-step explanation:
Here, we want to calculate the mean life time
From the question, 95% of the bulbs die before 920 hours
What this mean is that the probability that a bulb will die before 920 hours is 95% = 95/100 = 0.95
Now, we need the z-score that is exactly equal to this value
Using the standard normal distribution table, the z-score corresponding to this probability value is 1.645
Mathematically;
z-score = (x-mean)/SD
In this case, x is 920, SD is standard deviation which is 920 hours
thus, we have it that;
1.645 = (920-mean)/110
110(1.645) = 920 - mean
180.95 = 920 - mean
mean = 920- 180.95
mean = 739.05
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