Probability is the chance that something will happen - how likely it is that some event will happen.

An operation which results in some well defined outcome is called experiment.

The set of all possible outcomes of a given experiment is called the sample space, and it is usually denoted by S.

A subset of the sample space is called an event.

Mutually Exclusive Events: Let S be a sample space , A and B are two events such that A∩B=φ ,then A and B are said to be mutually exclusive events of sample space S.

Equally Likely Evnts: Two events A and B of a sample space S are said to be equally likely events if each one of them has number of chances of occurrence.