Generally, the chance of rolling at least one 1 on N D6 is 1 - (the chance of rolling no 1s at all). The latter term is (chance of rolling no 1 on die 1) * (chance of rolling no 1 on die 2) * ... * (chance of rolling no 1 on die N) = (5/6)
N
So you end up with 1 - (5/6)
N
In the case of N=2 this is equal to 1 - 25/36 = (36-25)/36 = 11/36, or what Crumb said.
