This is a hypergeometric calculator, an invaluable tool in deckbuilding to decide ratios of cards.
For Cards Drawn, enter 4 + amount of mulligans (on average 2) + the turn # you want the card by.
For Cards Wanted, enter the number of copies of card you're looking for.
Example: Finding 1 of 3 fioras by turn 3. Cards Wanted: 3 (3x fioras), Cards Drawn = 11 (4 (starting hand size) + 4 (full mulligan) + 3 (turn 3))