In this current scenario,
Probability of passing, p = 65% = 0.65
Then,
Probability of not passing, q = 1-p = 1-0.65 = 0.35
Part (a): When 15 people are tested
(i) Number of people expected to pass
This is 65% of the 15 people tested. That is,
Number of people expected to pass = 0.65*15 = 9.75. This is rounded downwards as upward rounding will violate the 65% criteria.
Therefore,
Number of people expected to pass = 9 people.
(ii) Probability that 11 people are expected to pass the test
p(x=11) = [15Cx]*p^x*q^(15-x) = [15C11]*0.65^11*0.35^(15-11) = 0.1792 ≈ 17.92%
Part (b): Teenager determined to pass the test no matter how many times
(i) Probability that he passes the test the third time
This means that he will fail the first and second time. That is,
Probability pf passing the third time = q*q*p = 0.35*0.35*0.65 = 0.079625 = 7.9625%
(ii) Number of trials it takes to pass
This is a case of mathematical expected, E, that it takes before first occurrence of success. Normally,
E = 1/p
Substituting;
E = 1/0.65 = 1.54 ≈ 2
Therefore, at least two trials will be required.