Exercise 2.12.  Define a constructor make-center-percent that takes a center 
and a percentage tolerance and produces the desired interval. You must also 
define a selector percent that produces the percentage tolerance for a given 
interval. The center selector is the same as the one shown above.

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(define (make-center-percent center tolerance)
  (let ((max-err (* (abs center) (/ tolerance 100))))
    (make-interval (- center max-err) (+ center max-err))))

(define (percent interval)
  (* 100
     (/ (width interval)
        (center interval))))