To calculate the monthly salary for 26 days in the month of September 2018 (25 days), you can use the following formula:
Monthly Salary = (Daily Rate x Number of Working Days) / Total Number of Days in the Month
In this case, the total number of days in September 2018 is 30. Therefore, assuming a fixed daily rate, you would multiply the daily rate by 26 (number of working days) and then divide the result by 30 to obtain the monthly salary.