Lisp Count Div
(defun count-div (a b k)
(let* ((first-div (if (zerop (mod a k)) a (+ a (- k (mod a k)))))
(last-div (- b (mod b k))))
(1+ (/ (- last-div first-div) k))))
This counts how many numbers in a range are divisible by K without looping through every value.