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Answer:
There is an 84% probability that your standard refrigerator will last between 12 and 20 years.
Step-by-step explanation:
Problems of normally distributed samples can be solved using the z-score formula.
In a set with mean and standard deviation
, the zscore of a measure X is given by:
The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.
In this problem, we have that:
A standard refrigerator's mean life expectancy is 18 years with a standard deviation of 2 years. This means that .
What is the probability that your standard refrigerator will last between 12 and 20 years?
This probability is the pvalue of Z when subtracted by the pvalue of Z when
. So
X = 20
has a pvalue of 0.8413
X = 12
has a pvalue of 0.0014
This means that there is a 0.8413-0.0014 = 0.8399 = 0.84 = 84% probability that your standard refrigerator will last between 12 and 20 years.