First off, rewrite all of the fractions so that they have a common denominator. That's 2/12, 2/12, and 4/12. Adding these together, we get 8/12. That's the probability of getting an A, B, or C — but not a D. The chance of getting a D is represented by the remaining 4/12 — or 1/3.
TIn short, the chance of rolling D is exactly 1/3.
