I hope this makes sense, if not please feel free to let me know and I'll explain it.
Answer:
The probability is 0.0428
Step-by-step explanation:
First, let's remember that the binomial distribution is given by the formula:
where k is the number of successes in n trials and p is the probability of success.
However, the problem tells us that when there isn't a number of trials fixed, we can use the geometric distribution and the formula for getting the first success on the xth trial becomes:

The problem asks us to find the probability of the first success on the 4th trial (given that the first subject to be a universal blood donor will be the fourth person selected)
Using this formula with the parameters given, we have:
p = 0.05
x = 4
Substituting these parameters in the formula and solving it, we get:

Therefore, the probability that the first subject to be a universal blood donor is the fourth person selected is 0.0428 or 4.28%
Answer:
x+1/x-1 +1 / x+1/x-1 -1= x
It's division.
Hope I helped!