Answer:
Finding the probability of a single die roll requires little effort.
First, remember that by definition, a prime is an integer with precisely two factors: 1 and the number itself (which is why 1 does not qualify: it has only one factor). The numbers from 6 on down that fit the bill are {2,3,5}. Since there are six sides, your answer is:
P(prime) = 3/6 = 1/2 = 50%
