Exact Solution Method

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