Answer:
the sample standard deviation
Explanation:
The statistic that would enable Dan to estimate the probability that any single class will last somewhere between 47 and 53 minutes is the sample standard deviation. If we have the mean and the standard deviation and we know that the lenghts of the classes form a normal curve, we can compute probabilities either with the empirical rule or using a table of probabilities from a book. The normal distribution is completely determined with the mean and the standard deviation.