For example, take a six-sided dice number 1-6 on each side. The probability of rolling a 4 would be1 in 6 or 1/6. This is because of the six possible outcomes, rolling a 4 is only one of them.
P(4)=1/6
Now take the same dice. The probability of rolling an even number is 3/6. To see why, lets look at the total possible outcomes:
1 2 3 4 5 6
Of the 6 possibilities, 3 are even numbers. P(even)=3/6 or 1/2