A company knows that 30% of Customers who come to the store will check out the merchandise and then order it online because it is cheaper. The company wants to know the probability that it will take at least three customers to find one who shops on line. How could the company find out this information?

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%