Final answer:
To estimate the expected (mean) time for activity B, use the provided probability and the z-score formula.
Step-by-step explanation:
To estimate the expected (mean) time for activity B, we can use the information provided. The probability is 0.7919 that the mean excess time used is more than 20 minutes for a sample of 80 customers. We can interpret this as the probability of observing a sample mean exceeding 20 minutes. From this, we can estimate the mean time for activity B by finding the value at which the cumulative probability exceeds 0.7919.
To find this value, we can use a standard normal distribution table or a calculator. Using the table, we look for the z-score that corresponds to a cumulative probability of 0.7919. In this case, the z-score is approximately 0.85.
Now, we can use the z-score formula to find the estimated mean time for activity B:
Estimated mean time for activity B = mean + (z-score)(standard deviation)