Card (I)- The number of I elements in the set.
P(A)= card (A)/ card (I)
For example, the probability of event A when the number of points where the dice landed is a multiple of 3.
P(A)= card (A)/ card (I)=2/6= 1/3.
Where Card(A)=2, because there are only two multiples of 3, 3 and 6.
Card (I)=6, because you will get 1, 2, 3, 4, 5, 6 * * when you roll the dice.
I hope you can understand.