Answer
True.
Work Step by Step
Theoretical probability applies to experiments in which the set of all equally likely outcomes, called the sample space, is known.
An event is any subset of the sample space.
The theoretical probability of event $E$ with sample space $S$ is
$P(E)=\displaystyle \frac{number\ of\ outcomes\ in\ E}{total\ number\ of\ possib1e\ outcomes}=\frac{n(E)}{n(S)}$
If there are n outcomes, each has probability $\displaystyle \frac{1}{n}$, and the sum of the probabilities of all the outcomes equals $n\displaystyle \cdot\frac{1}{n}=1.$
The statement is true.
