Here we have to assume that the grade is decided only from the final exam and the course right before the exam.
Final exam count 30% of the grade.
Then the course counts 70% of the grade.
Sally's final mark as of 100 marks `= 78.5`
Sally's final mark as of 70 marks `= 78.5/100xx70 = 54.95`
At the moment she has already taken 54.95% of final grade by doing course only. So she don't need any mark for final exam.