#| BEGIN (Write your solution here) |#
(define (double x) (* x 2))
(define (halve x) (/ x 2))

(define (fast-mul a b)
  (cond ((= b 0) 0)
        ((even? b) (double (fast-mul a (halve b))))
        (else (+ a (fast-mul a (- b 1))))))
#| END |#