Emacs Lisp function to calculate a test result

calculate.test-result.el

(defun calculate-test-result ()
  "Calculate test result.
This function assumes that you use Y for correct answers, and N for wrong ones."
  (interactive)
  (let* ((correct (how-many "Y" 1))
	 (wrong (how-many "N" 1))
	 (total (+ correct wrong))
	 (percentage (/ (* 100.0 correct) 
                        total)))
    (message (concat "Your test score percentage: "
		     (number-to-string percentage)))))

(global-set-key (kbd "<f12>") 'calculate-test-result)