21X

From BoardGameGeek:

21X is an algebraic blackjack game in which most cards have an X in their value, with you getting to choose an integer value for X that makes your card total as close to 21 as possible without going over. What's the best hand you can make when your cards are 24/X and 6-4X? Some cards have fixed values, and others contain an N variable that equals the number of cards you have in front of you.