Your statistics teacher tells you that there is a 50% chance that a coin will land heads up. How would you state this chance in terms of a probability?
IF we have a fair coin there are two possibilities:
heads or tails
Both have an equal chance.
So you divide the favourable chances ("success")
What is the chance of rolling less than three with a normal die?
Almost no real-life coin is completely fair.
Depending on the faces of the heads and tail, the centre of gravity may be a tiny bit on the heads or tail side.
This will only show on long term mega-flipping, but this has been done! Google!