### Video Transcript

If the probability of an event
happening is 11 out of 30 or eleven thirtieths, what is the probability of the event
not happening?

We know that the probability of an
event not happening is known as the complementary event. This is denoted by 𝐴 bar or 𝐴
prime, where the probability of 𝐴 bar is equal to one minus the probability of
𝐴. In this question, we are told the
probability of the event happening is 11 out of 30. If we let this be event 𝐴, then
the probability of 𝐴 is equal to eleven thirtieths or 11 out of 30. We can then calculate the
probability of the event not happening by subtracting this from one. One whole one is the same as 30 out
of 30 or thirty thirtieths. As the denominators are the same,
we simply subtract the numerators, giving us 19 out of 30 or nineteen
thirtieths. The probability of the event not
happening is therefore equal to 19 out of 30.