Suppose that you are rolling a six sided dice.

Let A = get a number smaller than 2

What is P(Ac)?

A die has 6 faces with the numbers 1,2,3,4,5 and 6 one on each of the six faces.Considering an unbaised die the probability of any of the faces to appear in a single roll is 1/6.
So,
P(1)=P(2)=P(3)=P(4)=P(5)=P(6)=1/6
Now that number smaller than 2 is "1"

P(A) = 1/6

P(Ac) = 1 - P(A) = 1 - ( 1/6 ) = 5/6