It depends on your course. There are separate exams which are held for theory and practicals. The system followed is that of, credits. Usually the practicals consist of 2 credits per practical and theory consists of either 4 credits or 3 credits. You are awarded grades in each of the subjects which have different scores allotted against each of them. The summation of the product of the credits and the grade you got in each subject divided by the summation of total credits gives you your CGPA. For example O grade is 10 points and you got O grade in a subject of 3 credits. A grade is of 8 points and you got A in a subject of 2 credits. Then your total Grade points will be (3*10 +2*8)/(3+2)= 9.2. Hope this helps. :)