"You buy the seventieth donkey."
Translation:Tu cumperi al șaptezecilea măgar.
EDIT: after a quick look on google translate (not the best i know) it seems to use măgarul, so I'd suggest reporting it. I am not a native Romanian speaker so I haven't a clue about what to use.
From reading other comments on other sentences in this section 'al șaptezecilea' means the seventieth and includes the definite article there so the unarticulated noun is used in conjunction. There is another way of saying it where you would use măgarul instead, but i can't remember exactly how those sentences are formed