Assume that you have a (fair) die  and the random experiment is
"The value facing up after one roll of the die".

You know there is an equally likely chance of rolling any of the values 1 through 6.
So...
The exact chance of rolling a  is 1/6     The exact chance of rolling a  is 1/6

So the exact chance of rolling a  or a  is 1/6 + 1/6 = 1/3