(define (sub-num? a b)
  (define (check-sub? num r factor counter)
    (if (= counter (num-of-digits a))
        (= a r)
        (check-sub? (quotient num 10) (+ r ( * factor (remainder num 10))) (* 10 factor) (+ 1 counter))))

  (define (number-iter n)
    (if (check-sub? n 0 1 0)
        #t
        (if (= n 0)
            #f
            (number-iter (quotient n 10)))))
  (number-iter b))