Let X be a discrete random variable with geometric distribution.
 Let x be the number of tests and p the probability of success in each trial, then the probability distribution is:
 P (X = x) = p * (1-p) ^ (x-1). With x = (1, 2, 3 ... n).
 This function measures the probability P of obtaining the first success at the x attempt.
 We need to know the probability of obtaining the first success at the third trial.
 Where a success is defined as a customer buying online.
 The probability of success in each trial is p = 0.3.
 So:
 P (X = 3) = 0.3 * (1-0.3) ^ (3-1)
 P (X = 3) = 0.147
 The probability of obtaining the first success at the third trial is 14.7%
