Answer:
14.46% probability that the instructor can finish grading in 4 and a half hours
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.
For sums of n values from a distribution, the mean is 
and the standard deviation is 
In this problem, we have that:
What is the probability that the instructor can finish grading in 4 and a half hours
Four and half hours is 4.5*60 = 270.
So this probability is the pvalue of Z when X = 270.
By the Central Limit Theorem
has a pvalue of 0.1446
14.46% probability that the instructor can finish grading in 4 and a half hours
