Answer:
For this case the average is an statistic unbiased for the population parameter

And the average is calculated with the following formula:

Where n represent the sample size
N represent the population size, and X the observations for this case the annual income.
This statistic is an unbiased estimator of the population mean since

After calculate the average they got:

And we can use the term "sample mean" instead of the average, and is a measure of central tendency for the data
Explanation:
For this case the average is an statistic unbiased for the population parameter

And the average is calculated with the following formula:

Where n represent the sample size
N represent the population size, and X the observations for this case the annual income.
This statistic is an unbiased estimator of the population mean since

After calculate the average they got:

And we can use the term "sample mean" instead of the average, and is a measure of central tendency for the data