Consult the chart that I have attached. This chart represents all the possible ways to roll 2 dice. The sum is written in each cell (eg: rolling a 2 and 3 yields a total of 2+3 = 5). There are 6*6 = 36 ways to roll the two dice. Of these 36 outcomes, only 13 of them have results that add up to a single digit prime number. The 13 different outcomes are
1+1 = 2
1+2 = 3
2+1 = 3
1+4 = 5
2+3 = 5
3+2 = 5
4+1 = 5
1+6 = 7
2+5 = 7
3+4 = 7
4+3 = 7
5+2 = 7
6+1 = 7
Divide the two values (13 and 36) to get: 13/36 = 0.3611 = 36.11% roughly
