This is a simple use of the multiplication rule of probability, which is:
"<em>The probability that Events A and B both occur is equal to the probability that Event A occurs times the probability that Event B occurs, given that A has occurred</em>"
Using this rule, we square 1/6 to get 1/36. In other forms:
Decimal:0.02777777777(Round to whatever decimal place you need)
Percentage: 2.778%(I don't see you needing any more places)
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.
