set 1:            set 2:
Joe,A,85,90       Joe,A,80,85
Joe,B,80,99       Joe,B,70,90
Rob,A,50,70       Rob,A,40,70
Rob,B,60,65       Tim,A,70,89
Tim,A,87,89       Lou,A,30,51
Jon,B,82,92
Lou,A,30,51

Output:
Joe,A,5,5
Joe,B,10,9
Rob,A,10,0
Rob,B,60,65
Tim,A,17,0
Jon,B,82,92
Lou,A,0,0