To solve questions where you need to find the probability of any event taking place, you first need to determine two values:
1. The number of ways in which the event can take place (a)
2. The total number of events that are possible (b)
Using these values, the probability of the event taking place is given by the value of a/b.
The value of the probability of anything happening or not happening always lies in the set [0, 1]. No event can have a probability less than 0 or more than 1. The probability of an event not taking place is equal to 1 - the probability of the event taking place.
Given the probability of two mutually exclusive events a and b taking place as P(a) and P(b), the probability that both take place is P(a)*P(b) and the probability that either of them takes place is P(a) + P(b).