Answer:
The blood potassium level L such that the probability is only 0.05 that the average of four measurements is less than L is 3.64.
Step-by-step explanation:
To solve this question, we have to understand the normal probability distribution and the central limit theorem.
Normal probability distribution:
Problems of normally distributed samples are 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.
Central limit theorem:
The Central Limit Theorem estabilishes that, for a random variable X, with mean and standard deviation , the sample means of size n can be approximated to a normal distribution with mean and standard deviation, which is also called standard error 
In this problem, we have that:
What is the blood potassium level L such that the probability is only 0.05 that the average of four measurements is less than L?
This is the value of X when Z has a pvalue of 0.05. So it is X when Z = -1.645.
By the Central Limit Theorem
The blood potassium level L such that the probability is only 0.05 that the average of four measurements is less than L is 3.64.
