SICP 2.12

(define (make-interval a b) (cons a b))
(define (lower-bound i) (car i))
(define (upper-bound i) (cdr i))

;;; c is the center
;;; n is the percentage
(define (make-center-percent c n)
  (let ((w (* c (/ n 100)))) ;;; convert percentage to width
    (make-center-width c w)))
(define (percent i)
    (* (/ (width i) (center i)) 100))
    
(define (make-center-width c w)
  (make-interval (- c w) (+ c w)))
(define (center i)
  (/ (+ (lower-bound i) (upper-bound i)) 2))
(define (width i)
  (/ (- (upper-bound i) (lower-bound i)) 2))