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

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

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

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