(define letter-to-number
  (lambda (letter)
    (if (eq? letter 'A)
	4.0
	(if (eq? letter 'B+)
	    3.5
	    3.0))))

(letter-to-number 'A)
(letter-to-number 'B+)
(letter-to-number 'B)

(define letter-to-number
  (lambda (letter)
    (cond
     [(eq? letter 'A) 4.0]
     [(eq? letter 'B+) 3.5]
     [else 3.0])))