i dont understand why before table can change to after table

Problem 9: [3, Exercise 2.16] The following table shows the distribution function of a random variable X. Determine (a) the probability function, (b) P(1 2), (d) P(X 1.4). 2 3 4 F(I) 1/8 3/8 3/4 1 Solution: a) 1 2 3 4 f(I) 1/8 1/4 3/8 1/4