This is not a purely mathematical problem. For those who dabble in probability, let me remind you that in practice very few coins are fair, let alone some that are purposely made to bias on one side. There is also a consideration on how the coin is tossed. A practiced individual can affect the outcome by the way he tosses the coin.
If you put aside the behavior of a theoretical coin and observe the coin tossing event, like the LZ said, it will become a statistical event. If I toss that coin 100 times and it turns up head 60 times, do you still believe that the probability of turning up head is half?