40 ILCS 5/13-208

"Average final salary": The highest average monthly salary as calculated by accumulating the salary for the highest 520 consecutive paid days of service within the last 10 years of service immediately preceding the date of retirement and dividing by 24. If the employee is paid for any portion of a work day, the fraction of the day worked and the salary for that fraction of the day shall be counted in accordance with the Fund's administrative rules.