The probability of an event and the probability of its complement always sum to:

Probability is defined as the ratio of the total number of favorable outcomes by the total number of possible outcomes.

Answer: The probability of an event and the probability of its complement always sum to 1.

Let us proceed step by step

Explanation:

The probability of any event is the possibility of any event. 

According to the complement rule, the sum of the probabilities of an event and its complement must be equal to 1.

Let us consider an event A

Then according to the compliment rule P(A) + P(A') = 1

Where P(A') is the complement of  P(A).

Therefore, the probability of an event and the probability of its complement always sum to 1.