This is usually written as a fraction, but if the event is certain then the probability is 1 and, if the event is impossible, the probability is 0.
Five counters numbered 1 to 5 are placed in a bag. If one counter is drawn out at random, what is the probability that it will be a counter with an even number?
From the numbers 1, 2, 3, 4, 5 there are just 2 even numbers out of 5 numbers.
The probability of getting an even number is 2/5