For the life of me I dont understand this.
each probability is given as x/10
so P(X=1) = 1/10 = 0.1
P(X=2) = 2/10 = 0.2
P(X=3) = 0.3
P(X=4) = 0.4
each of these lies between 0 and 1. So that probability is good.
Finally to form a Prob Function - Sum of all possible probabilities should be 1.
which it is. 0.1 + 0.2 + 0.3 + 0.4 = 1
so - it is a good probability function.