Probability of the outcome you're looking for =
(number of ways your outcome can happen)
divided by
(total number of possible outcomes) .
Total number of possible outcomes when a single fair is tossed = 6 .
The outcomes you're looking for: 5 or 6 . (two of them) .
Probability = 2 / 6 = 1/3 = 33-1/3 %