;Удаление нечётных элеметнтов
(define (remove_odd_tail lst i)
  (cond
    [(null? lst) '()]
    [(odd? i) (cons (car lst) (remove_odd_tail (cdr lst) (+ i 1)))]
    [else (remove_odd_tail (cdr lst) (+ i 1))]
  )
)
(remove_odd_tail (list 10 20 30 40 50 60 70 80 90 100) 0)