calculating a penalty for late submited coursework for students

i am making a spread sheet table to auto decide whether a piece of coursework is late and who to decide how much penalty to allocate

i have got the total mar in percentage

the submission date for each student

deadline date which is 05/05/2009 located in cell G1

each student is allowed up to 3 weeks to be late each week being a 10% penalty and a total mark of 0 after the 3rd week.

i have attached a copy of the table pre filled with dummy data

thank you