#| BEGIN (Write your solution here) |#
(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)
  (car (cdr mobile)))

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

(define (branch-structure branch)
  (car (cdr branch)))

(define (total-weight mobile)
  (if (not (pair? mobile))
      mobile
      (+ (total-weight (branch-structure (left-branch mobile)))
         (total-weight (branch-structure (right-branch mobile))))))

(define (mobile-balanced? mobile)
  (if (not (pair? mobile))
      #t
      (let ((left-torque (* (branch-length (left-branch mobile))
                            (total-weight (branch-structure (left-branch mobile)))))
            (right-torque (* (branch-length (right-branch mobile))
                             (total-weight (branch-structure (right-branch mobile))))))
        (and (= left-torque right-torque)
             (mobile-balanced? (branch-structure (left-branch mobile)))
             (mobile-balanced? (branch-structure (right-branch mobile)))))))
#| END |#