# `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)