Not sure how to go about this question. Would the sample mean just be 200/4 = 50 and then tauhat

7.28 A certain bank has 20,000 home mortgages numbered serially in the order that they were granted, over a period of years. There is a tendency for unpaid balances to in- crease from the lowest to highest number on the list because housing costs have been rising. It is desired to estimate the total unpaid balance. A sample of size 200 is to be used, and four clerks are available to do the work. How would you design the sam- pling scheme and estimate the desired total? Construct an approximate variance from your estimator