Martin's economics teacher puts a grade of a for an average of 90 or more; of b for 80 or more. of the four exams of the course, martin has presented the first with a grade of 82; the second, gets a grade of 75; and the third with a grade of 71. if the final exam is twice the average of the previous ones, what grade should martin obtain to achieve an average of a? of b? (suppose the maximum possible rating is 100)