A difficult assembly process must be undertaken, and the probability of success at each attempt is 0.2. The distribution of the number of independent attempts needed to achieve success is given by the product rule as

Plot the distribution function and find the probabilities that the number of attempts will be

(a) Less than four;

(b) Between three and five.

P(X= k) = (0.2)(0.8) (k = 1, 2, 3, ...)