Рекурсия

(defun balance (w &aux (m (/ (reduce #'+ w) (length w))))
  (loop for a in w 
        if (< a m) collect a into b
        else if (> a m) collect a into c
        finally (return (list b m c))))
 
> (balance '(1 2 3 4 5))
((1 2) 3 (4 5))

(defun balance (w &optional b (m (/ (reduce #'+ w) (length w))) c)
  (cond ((null w) (list (reverse b) m (reverse c)))
        ((< (car w) m) (balance (cdr w) (cons (car w) b) m c))
        ((> (car w) m) (balance (cdr w) b m (cons (car w) c)))
        (t (balance (cdr w) b m c))))
 
> (balance '(1 2 3 4 5))
((1 2) 3 (4 5))

(defun balance (w &optional b (m (/ (reduce #'+ w) (length w))) c)
  (cond ((null w) (list (sort b #'<) m (sort c #'<)))
        ((< (car w) m) (balance (cdr w) (cons (car w) b) m c))
        ((> (car w) m) (balance (cdr w) b m (cons (car w) c)))
        (t (balance (cdr w) b m c))))
 
> (balance '(1 2 3 4 5))
((1 2) 3 (4 5))
> (balance '(2 5 3 1 4))
((1 2) 3 (4 5))

> (balance '(1 1 1 1 1 2))
((1 1 1 1 1) 7/6 (2))