Total number of possible outcomes (faces on the cube) = 6 .
The prime numbers are 2, 3, and 5.
The composite numbers are 4 and 6.
(I think '1' is not included in either category.)
There are 2 composite numbers on the cube, out of 6 faces.
So the probability of rolling one is
2/6 = 1/3 = 33 and 1/3 % .