There are 6 sides on a dice. This means each number has a 1/6 chance of appearing. (1 number out of 6 sides). This includes 6, so the answer to a is 1/6. If you roll or do not roll a six, this means every side of the dice counts. The answer to be, by extension, is 6/6. Finally, c is b (the probability of anything) minus a (the probability of a six), representing every situation that is not a 6, so 5/6.
