; First, possible mobile and branch abstractions:

(define make-mobile cons)
(define left-branch car)
(define right-branch cdr)

(define make-branch cons)
(define len car)
(define struct cdr)

(define weight? number?)
(define mobile? pair?)

; Here is a test mobile so we can have a sample input to the
; programs, below.  Note that we must adhere to the abstractions.

(define m
   (make-branch 6
     (make-branch 1 8)
     (make-branch 4 2)))
   (make-branch 5 12)))

; OK, now check out the elegance of the program.  (This is the
; part that really appeals to the mathematician in me.  Yeah,
; so that may seem weird, but you should be used to me by now.)
; The torque of a branch is its length times its total weight.
; To get the total weight of a branch, it's either going to be
; a single weight or the total weight of the mobile hanging off
; that branch...

(define (branch-weight branch)
   (let ((thing (struct branch)))     ; to minimize computation
      (cond ((weight? thing) thing)   ; the thing is a weight
            (else (mobile-weight thing)))))  ; thing is a mobile

; And we have a great case for mutual recursion because the
; weight of a mobile is simply the sum of the weights of its
; branches.  And we already have a function that figures out
; the weight of a branch, above.

(define (mobile-weight mobile)
   (+ (branch-weight (left-branch mobile))
      (branch-weight (right-branch mobile))))
; And now the beautiful algorithm.  First, torque, which is
; the length of a branch times its weight...

(define (torque branch)
   (* (len branch) (branch-weight branch)))
; And now balanced?  The base case is essentially a weight.
; If a weight is reached, it's balanced.  If a mobile is
; reached, then if its left side torque equals its right
; side torque AND any sub-mobiles of the mobile are also
; balanced, this predicate should return #t.  Otherwise, it
; should return #f.

(define (balanced? mobile)
   (or (weight? mobile)
       (and (= (torque (left-branch mobile))
               (torque (right-branch mobile)))
            (balanced? (struct (left-branch mobile)))
            (balanced? (struct (right-branch mobile))))))

; Eh?  
; Certainly, this could have been written with an if or a cond,
; but it turns out there is no need.
; If the mobile is a weight, the first part of the or will be #t
; and Scheme's or is a special form, using normal-order evaluation.
; Note that it makes sense to consider the possibility of a mobile
; simply being a weight.  A weight isn't going to tilt left or
; right.  It will simply sit there.  That's as balanced as it gets.
; If the mobile is not a weight, then the and will be computed.
; The and will not bother to compute either of the balanced? predicates
; if the torques are not equal due to normal-order evaluation.
; There are other ways to approach this problem, but this is as
; close as I can come to translating a problem description in
; English into code in any programming language.