Greater than less than Formula

I'm trying to create a spreadsheet to track my employees hours, straight time versus overtime. in one of my columns, I'm trying to have it auto count where if the total hours worked throughout a certain amount of cells exceeds 40, it will count how many hours it was over 40. for instance, if someone worked 56 hours, I'm trying to get the overtime cell to be able to add just 16 in the box to tell me how many hours over 40 were worked for the certain employee the week and then have a sum box at the bottom of my sheet for the totals for all the employees. i keep playing with "countif" and "greater than and less than" and ive wasted so much time trying to find this type of formula. can someone please help me!?!?!