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Witty, it's an average of your grades (marks) in school (university). An A grade counts as four points, B three, C two, D one and F zero. Each class is worth a certain number of credits.
For instance, your history class might be worth three credits and your art class is worth one credit. Let's say you got an A in history and a C in art. Your GPA would be 3.5 because you'd have 14 points (12 from the three-credit A and two from the one-credit C) divided by four total credits.