Probability is nothing but the chance of occurring
probability scale is shown below
Sum of all probabilities in sample space equal to one
the probability of an event which can not occur is zero
the probability of an event which must cour is one
the the the probability of the event not occurring will be one minus the probability of not occurring
If A and B are mutually exclusive events then.
P(A or B)=P(A)+P(B)
The complement of an event is denoted as E' which can be written as P(E')=1-P(E)