Answer:
On the first day, 98.75% probability that the sample average amount of time taken is at most 11 min.
On the second day, 99.29% probability that the sample average amount of time taken is at most 11 min.
Step-by-step explanation:
To solve this question, we need 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 normally distributed random variable X, with mean and standard deviation
, the sampling distribution of the sample means with size n can be approximated to a normal distribution with mean
and standard deviation
.
For a skewed variable, the Central Limit Theorem can also be applied, as long as n is at least 30.
In this problem, we have that:
First day
The probability is the pvalue of Z when X = 11. So
By the Central Limit Theorem
has a pvalue of 0.9875
On the first day, 98.75% probability that the sample average amount of time taken is at most 11 min.
Second day
The probability is the pvalue of Z when X = 11. So
By the Central Limit Theorem
has a pvalue of 0.9929
On the second day, 99.29% probability that the sample average amount of time taken is at most 11 min.