This is just like the game blackjack, but with a ten-sided die. The goal of the game is simple. Get to 21, but not over. Roll the die. The number on the die is added to your score. You can hold, which means you roll again but the number is not added to your score. If the die gets you over 21 after you have held, you win. If you hold and get under 21 or 21, you lose. The calculator tells you the risk of rolling again.
I'd like to thank minecraft185 for the idea. I'd also like to thank mantisshrimp for the risk calculator idea.