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.

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.