The numbers on a die which are either odd or prime are these four:
1,2,3, and 5
All the numbers on a die are these six:
1,2,3,4,5, and 6.
So there are 4 ways out of 6 that you can roll an odd or prime number.
When calculating probabilities, "out of" means "over" or "divided by.
So 4 ways out of 6 means there is a probability of "4 over 6" or a
probability of 4/6 which reduces to a probability of 2/3. That means
that two-thirds if the time when you roll a die it will land either
an odd number or a prime number.
[or both if you happen to roll 3 or 5.]
Edwin