Your current grade and final-exam score do not usually count equally. The syllabus weight determines how much the exam can move the result.
Use decimal weights
Convert a 25% final to 0.25. Multiply the current grade by 0.75 and the exam score by 0.25, then add the contributions. An 80 current grade and a 90 exam score produce 82.5.
Try different outcomes with the Final Grade Calculator.