A dice has numbers 1 through 6. Prime numbers are numbers that cannot be divided by anything except itself and one. Remember that 1 is not a prime number.
That means the numbers on a dice that are prime are: 2, 3, and 5. That means 3 out of the 6 numbers are prime, so the probability of rolling a prime number, since all sides have the same chance of being rolled is:
Your answer is 1/2 probability of rolling a prime.