We will assume that the 8-sided die has the numbers :
1 2 3 4 5 6 7 8
The prime numbers are : 2 3 5 7
Then the outcomes are 2 3 5 7
The probability of having a prime number if the die rolled once= 4/8=1/2.
The odds of the outcome if an 8 sided dice is rolled.
The odds of the outcome if you roll a dice of 8 sides each with a number 1 to 8 are : One of the 8 faces coming up.
The odds in favour of a prime is 5 out of 8 , the primes are (1,2,3,5,and7).
So the probability of the odds in favour of prime is 5/8 in a rolling
It is assumed that the dice is marked with numbers 1 to 8, and the probability of getting any one of these number on a roll is equal.
That is probability of getting any number from 1 to 8 on a roll of dice = 1/8
There are four prime numbers between 1 and 8. These are 2, 3, 5, and 7.
The probability of getting a specific of these 4 prime number on a roll = 1/8
The probability of getting any one of the 4 prime numbers on a roll:
= Sum of probabilities of getting each of the four numbers
= 1/8 + 1/8 + 1/8 + 1/8 = 4/8 = 0.5
In other words odds of getting a prime number versus not getting a prime number are 1:1.