Answer:

Step-by-step explanation:
Given
Let the probability of false positive be represented with p

Required
Determine the probability the first occurrence of p was at the third person
We have that:

The probability that a trustworthy person do not fail the test (q) is:



The required probability implies that:
The first person did not fail: i.e. 0.85
The second person did not fail: i.e. 0.85
The third person failed: i.e. 0.15
This is then calculated as:


<em>Hence, the probability that the first false positive occurred at the third person is 0.108375</em>