The total number of possible outcomes when rolling two standard six-sided dice is 6 x 6 = 36. To find the number of outcomes that result in a sum of 9, we can create a table to visualize all of the possible outcomes:
| Die 1 | Die 2 | Sum |
|:------:|:------:|:------:|
| 3 | 6 | 9 |
| 4 | 5 | 9 |
| 5 | 4 | 9 |
| 6 | 3 | 9 |
From this table, we can see that there are four possible outcomes that would result in a sum of 9. Therefore, the probability of rolling a sum of 9 with two standard six-sided dice is:
P(D1 + D2 = 9) = number of outcomes that result in a sum of 9 / total number of possible outcomes
P(D1 + D2 = 9) = 4 / 36
P(D1 + D2 = 9) = 1 / 9
So the probability of rolling a sum of 9 with two standard six-sided dice is 1/9.