cal. attendance percentage using datagridview c# winforms

Question

hai friends,

 please  help me in calculating the attendance percentage and also the total no. of classes 
 
 in datagridview using c# winforms
these attendance  values are retrieving from database. the last two fields tot classes and percentage should calculate upto the attendance entered only  .

for example :   for 30 days  we posted attendance only for 10 days  then only 10days attendance(tot classes and percentage) only to be  calculated.

and  absent 'a' or 'A'  should indicate with red color. and attendance percentage less than 65 also to be marked red.

REGD_NO	NAME	MON	1	2	3	4	TOT CLASSES	%
1	CHIRU	JAN	1	A	2	3	3	75
2	VENU	JAN	A	A	1	2	2	50