If two events are complementary, then the sum of their probabilities is

The sum of 2 complementary probabilities is equal to ONE:

Let roll a dye, the probability of getting, sat number 4==> P (getting 4) = 1/6

Ant the probability of getting (either, 1 or 2 or 3 or 5 or 6) = = > P (1,2,3,5,6) = 5/6

5/6 is the complement of 1/6==> P (4) + P (1,2,3,5,6) = 1/6 + 5/6 = 1

The sum of the probabilities of complementary events are 1 or 100% if you need it in percents.

Ex. P (rain) =.70; P (not rain) =.30

.70 +.30 = 1