#lang racket
; number 11
(define(list-of-nums? lst)
  (cond[(empty? lst) '()]
       [(=(length lst) 1) (car lst)]
       [else(number?(car lst)) (list-of-nums? (cdr lst))]))

(define (sorted? lst)
  (cond[(empty? lst) '()]
       [(=(length lst) 1) (car lst)]
       [else(>(car lst) (second lst)) (sorted? (cdr lst))]))

(define/contract (sort-descending lst)
  (->(and/c list? list-of-nums?) (and/c sorted? list?))
     (sort lst (λ(x y) (> x y))))

(sort-descending '(1 99 3 2 45 6 -1 5 0 2))

; number 14
(define (sum-elems-divis-by-three lst)
  (define (helper acc lst)
    (cond[(empty? lst) acc]
         [(=(remainder (car lst)3)0) (helper (+ acc(car lst)) (cdr lst))]
         [else (helper acc (cdr lst))]))
  (helper 0 lst))

(sum-elems-divis-by-three '())
(sum-elems-divis-by-three '(1 3 6))
(sum-elems-divis-by-three '(2 3 4 5 6 7 8 9))