It's basically the total chances as the denominator and the amount of that odd being rolled. To make more sense, say there were we 3 green marbles and 3 red marbles. The chances of getting a green marble would be 3/6. So, in this case, since a cube (dice) has up to 6 numbers, there are 6 numbers it can land on and out of those, only one number is 5 so, the chances of a 5 being rolled is 1/6.