[Agent Minivann]

I don't know how to do it with math, but the probability is basically just figuring out how many of each possible combination of die rolls would be a success out of the total possible combinations. Your total combinations will be 240 (20 x 12). The easiest way to do it without figuring out the actual mathematic "short cut" would be to go into a spreadsheet and start with 1-20, twelve times each in the first column, then 1-12 repeating for each 1, 2, etc in the second column. The third column could then have a 1 for each instance where the attack wins, the fourth column has a 1 for each instance where the defense wins. Then sum the third and fourth columns and divide by 240. I'm sure there are easier ways to do it, but I don't know them.