Pay sales tax with the Retail Sales Tax Return For Occasional Sales (DR 0100A) on the tangible personal property included in the purchase of the business by the 20th day of the month following the sale.
So, to avoid pregnancy she has to better avoid unprotected sexual intercourse in this period. The rest of the days i.e., starting from menstruation to the 10th day and again from 21st day up to menstruation are the safe days or the safe period to avoid pregnancy.
To calculate safe and unsafe dates. If a women's menstrual cycle varies from 26days to 31days cycle, The shortest cycle [26days] minus18days=8th day. The longest cycle [31days] minus 10days=21st day Thus, 8th to 21st day of each cycle counting from first day of menstrual period is considered as fertile period.