The size of the bookstore is classified as Small, Medium and Large. A double sampling was performed to estimate the total sales of a certain book last month in the center county region, which has 500 bookstores. In the 100 bookstores selected randomly in the first phase, 50 are small, 30 are medium and 20 are large. Suppose a stratified random sample of 10 small, 6 medium and 4 large bookstores are carried out, the sales of the book title are:


Small 50 30 34 45 60 32 28 33 40 45

Medium 60 90 80 72 94 86

Large 210 150 186 140


Required:

Estimate the total sales of the book title in this region and estimate its standard deviation( also called the standard error of the estimate)