Can someone please help me get unstuck on this exercise, i dont know where to begin.

A simple random sample of 40 invoices from a small local shop (of 40 clients served) is drawn. The population is set of 300 bills for the months. For each bill, we noted x as the number of people served (since several local families shop for their kids here) and y as the bill amount. Here's a summary of the sample:

-Number of people served per bill (x): sum= 80 and corrected variance= 1,47

-Bill amount (y): sum=2699,11 and corrected variance of 1876,84

Assuming the number of invoices has not changed and remains at 300.

1-Determine the size of the sample that will need to be drawn for the margin of error in the estimate of total item for the month to be 4000$.

2-Determine the size of the sample that will need to be drawn so that the margin of error in the estimate of the total items for the month is 5%. (we estimate assuming that these parameters don't change).