(define (make-mobile left right)
  (list left right))

(define (make-branch length structure)
  (list length structure))

(define (left-branch mobile)
  (car mobile))

(define (right-branch mobile)
  (cadr mobile))

(define (branch-length branch)
  (car branch))

(define (branch-structure branch)
  (cadr branch))

(define (total-weight mobile)
  (if (number? mobile) ; mobile is a single weight
      mobile
      (+ (total-weight (branch-structure (left-branch mobile)))
         (total-weight (branch-structure (right-branch mobile))))))

(define (balanced? mobile)
  (define (branch-torque branch)
    (* (branch-length branch)
       (total-weight (branch-structure branch))))
  (if (number? mobile)
      #t
      (= (branch-torque (left-branch mobile))
         (branch-torque (right-branch mobile)))))

; 1 ]=> (define m (make-mobile (make-branch 2 (make-mobile (make-branch 2 1) (make-branch 1 2))) (make-branch 3 2)))
; 
; ;Value: m
; 
; 1 ]=> (balanced? m)
; 
; ;Value: #t
; 
; 1 ]=> (balanced? (make-mobile (make-branch 6 7) (make-branch 6 8)))
; 
; ;Value: #f

; ; If the constructors change to:
; (define (make-mobile left right)
;   (cons left right))
; 
; (define (make-branch length structure)
;   (cons length structure))
; ; then only the folloing selectors have to change
; (define (right-branch mobile)
;   (cdr mobile))
; 
; (define (branch-structure branch)
;   (cdr branch))