(define grade-number->letter
  (lambda (grade)
    (if (>= grade 93) 'A
        (if (>= grade 90) 'A-
            (if (>= grade 87) 'B+
                'F)))))

(define g-n->l
  (lambda (grade)
    (cond
      ((>= grade 93) 'A)
      ((>= grade 90) 'A-)
      (else 'F))))

(define table-lookup
  (lambda (grade)
    (list-ref (list 'F 'D 'C 'B 'A) (inexact->exact (round grade)) )))