There are really only 2 possible outcomes when flipping a coin: it lands heads up or tails up. So, if all possible outcomes' probability equals 100%, we can assume both are 50% likely.
Because of this, the theoretical probability that the coin will land tails up is 50%
Additionally, if 50% (or 1/2) of 250 people win (by landing tails up), you can simply divide by 2 to get that number of people: 125
Hope this helps :)
