Fill in the blanks:
The sum of probabilities of all the outcomes of an experiment is ___ .

Sum of the probabilities of each outcome in an experiment is $1.$

The set of all possible outcomes of an experiment is called the sample space of that experiment. The sum of probabilities of all the outcomes within a sample space is $1.$

This means, for example, that if an experiment can have three possible outcomes $(A,B,$ and $C),$ then $P(A)+P(B)+P(C)=1.$