We don't know how big to make the piles. The only number given in the problem is $30$, so let's try that: suppose we make a pile with $30$ cards (without turning over any card yet). It will have some number of face-down cards, say $k$, and the rest of the cards ($30-k$ of them) will be face-up.
The other pile has the remaining cards. This includes the rest of the original face-down cards (there were $30$ total, and some number $k$ are in the first-pile, so there are $30-k$ here), and some unknown number of face-up cards.
To summarize: the first pile has $k$ face-down cards and $30-k$ face-up cards; the second pile has $30-k$ face-down cards and some number of face-up cards.
Now we want to make the number of face-up cards equal in the two piles. It should be obvious how to achieve that from here. :-)
So yes, the problem can be solved for any number of face-down cards in place of $30$, provided we are told what the number is. We don't need to be told what the number of face-up cards is.