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Probability is a measure of the likelihood that a specific event will occur, expressed as a number between 0 and 1. An event with a probability of 0 means it will not happen, while a probability of 1 indicates certainty. Understanding how to calculate the probability of an event is fundamental in statistics, as it allows for predictions and informed decision-making.

Complementary events are two outcomes of a single event where one event occurs if and only if the other does not. For any event E, the probability of its complement E' (the event not happening) plus the probability of E must equal 1. This relationship is crucial for calculating probabilities, as it allows us to find the probability of an event by subtracting the probability of its complement from 1.

To find the probability of an event occurring when given the probability of it not occurring, you can use the formula P(E) = 1 - P(E'). In the context of the question, if P(E') = 3/4, then P(E) can be calculated as 1 - 3/4, which equals 1/4. This method is a straightforward approach to solving probability problems involving complementary events.