Here's the question:

a Problem 3. The size of the bookstore is classied as Small, Medium and Large. A double sampling was performed to esti mate the total sales of a certain book last month in the center county region, which has 500 bookstores. In the 100 book stores selected randomly in the rst phase, 50 are small, 30 are medium and 20 are large. Suppose a stratied random sample of 10 small, 6 medium and 4 large bookstores are carried out, the sales of the book title are: mum \"Emma-II- ---------- Estimate the total sales of the book title in this region and estimate its standard deviation( also called the standard error of the estimate)