Here is the binomial rule:

If  is the probability of getting  success in  trial,
and  is the number of trials, and , then the 
probability of getting exactly  successes out of the  
trials is



Here a success will be a head on a coin, and a trial
will be a toss. Here  is the probability 
of getting  head in  trial. There are 
trials (tosses), so . Then the probability of 
getting exactly  heads out of the  trials 
is gotten by substituting in that expression, with 
.








Edwin