So the point of the game I have made up is get to 21 by rolling two dice. On the first roll you have a 100% chance of rolling a sum less than 21, a 0% chance of rolling a 21, and a 0% chance to roll over 21.

If I roll again, what are the chances of getting a sum (when added with the first) lower, higher or exactly 21?

How would I go about calculating this?

How would I calculate this considering that the players may roll multiple times.

Thanks in advance