"The sixtieth is an ox."

Translation: Al șaizecilea este un bou.

[Roxot]

So when does the "i" at the end of "zeci" change into an "e" as in "al nouăzecelea" and when does it stay an "i"?

[splittongue]

Short answer: never.
See Lurch's answer above, and also my comment here: https://forum.duolingo.com/comment/29615107