The sum of probabilities of all outcomes in an experiment is__.

Probability of an event, P(E) = number of favourable outcomestotal/number of outcomes
Let's use tossing a coin as an example. The possible outcomes of this experiment are {H, T}. That is, the total number of outcomes is 2.
Probability of getting a head, P(H) = 1/2
Probability of getting a tail, P(T) = 1/2
P(H) + P(T) = 1/2 + 1/2=1
⇒ The probability of every result in an experiment added together is 1.