How do you write probability?

Generally you should express it as a number between 0 (impossible) and 1 (absolute certainty). Either as a decimal or fraction is OK. For example, the probability of a fair coin toss landing on heads is 0.5. The probability of a fair die landing on 4 is 1/6.

But it's also common to see probability written as a percentage between 0% (impossible) and 100% (absolute certainty). So P(coin landing on heads)=50.0% and P(die landing on 4)=16.7%.