Silly bottles problem

bottles.scm

;; You've just been hired at Lighthouse Markets, a reputable (and
;; thoroughly fictional) grocery store chain. One of the primary
;; features of Lighthouse Markets is their recycling program on pop
;; bottles. Here is how the program works:

;; * For every two empty bottles, you can get one free (full) bottle of pop
;; * For every four bottle caps, you can get one free (full) bottle of pop
;; * Each bottle of pop costs $2 to purchase

;; Given these parameters, write a program so that you can figure out
;; how many total bottles of pop can be redeemed given a customer
;; investment.

(require-extension numbers)

(define (bottles n)
  (define (iter empties caps acc)
    (if (and (< empties 2)
             (< caps 4))
        acc
        (let ((new-bottles (+ (quotient empties 2)
                              (quotient caps 4))))
          (iter (+ new-bottles (modulo empties 2))
                (+ new-bottles (modulo caps 4))
                (+ new-bottles acc)))))
  (iter n n n))