Probability for discrete events is number of elements (events) that a condition holds for, times the probability of the event, divided by total weight. Total weight of that matrix is 1.
For example, the entire first row is associated with X = 1, and 1 <= 1 1/2, so take the weights of the first row, 1/12 + 1/16 + 1/12 = 1/6 . None of the entries in the second or third row matches, so nothing is added for those rows. Total probability is (1/6) divided by (1) where the (1) is the total weight for the matrix.