This is a true statement.
Consider two events A and B. We say they are complementary if P(A)+P(B) = 1
This means that either event A or event B must happen, since the "1" represents 100% probability. Having a probability of 100% means absolute certainty.
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Example:
A = the event it rains
B = the event it does not rain
P(A) = 0.30 = 30% chance of rain
P(B) = 0.70 = 70% chance it does not rain
P(A)+P(B) = 0.30+0.70 = 1
So this shows the two events are complementary.