Example 7 A fair coin is tossed twice. What is the probability of getting two ‘heads'?

           Here and .
           Total number of outcomes is 4 and all four outcomes are equally likely.

           Only outcome favourable to is {HH}

                                           
Discussion

• The classical definition is limited to a random experiment which has only a finite number of outcomes. In many experiments like that in the above examples, the sample space is finite and each outcome may be assumed ‘equally likely.'  In such cases, the counting method can be used to compute probabilities of events.
                                
• Consider the experiment of tossing a fair coin until a ‘head' appears.As we have discussed earlier, there are countably infinite outcomes. Can you believe that all these outcomes are equally likely?
                       
• The notion of equally likely is important here. Equally likely means equally probable. Thus this definition presupposes that all events occur with equal probability . Thus the definition includes a concept to be defined.