17.16 Mean household income must be estimated for a town that can be divided into three districts. The relevant information is shown in the table.

District Population Size Estimated Standard Deviation ($)

1 1,150 4,000 2 2,120 6,000 3 930 8,000 If a 95% confidence interval for the population mean extending $500 on each side of the sample estimate is required, determine how many sample observations in total are needed under proportional allocation and optimal allocation.