Originally Posted by paasikiviSorry I messed up. Plural genitive of "неделя" is "НЕДЕЛЬ". So mainly I wondered why in this case it was plural genitive (instead of accusative) but in
"я бывал в пяти странах"
it is in plural prepositional. In both cases there is a numeral.
And I suppose also in
"она приехала со своими двадцатью пятью кошками"
the noun turns into instrumental, not genitive.