If I understand your question correctly then it's simple all you have to do is use the numbers in the place of the b and c so the question would be what is 8-7 which equals 1
100% of the rolls would be less than 13. The highest number on one die is 6, so even if you roll 2 dice the sum of both dice will always be lower than 13.
It's a 1:1 probablility.
The answer to the question is 50,000