ex-it

ex-it: string -> string

Change every “x” to a “y”.

(check-expect (ex-it "tresxxx") "tres")
(check-expect (ex-it "zyzzyva") "zyzzyva") ; no change

unlucky

unlucky: number -> number

How many multiples of 13 appear in the digits of n? (Just consider two digit multiples.)

(check-expect (unlucky 131) 1)
(check-expect (unlucky 139) 2)
(check-expect (unlucky 1263) 1)
(check-expect (unlucky 1326) 2)