This Exam P sample reference tests Hypergeometric Distribution. This is a two-case hypergeometric acceptance probability. An accepted sample contains either two good items and one defective item or three good items; the two cases each have 56 favorable samples, giving 112/120=14/15≈0.933 and choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe rejection probability is 1/15≈0.0667. Rounding that probability to one decimal as 0.10 and reporting it without complementing reverses the event.
BThe value 0.47 is one of the two acceptance components, 56/120. It omits the equally large all-good sample component.
CThe value 0.70 results from counting the two-good component fully but counting only half of the equally numerous all-good component: (56+28)/120.
DThe value 0.90 is the with-replacement binomial approximation Pr(Binomial(3,0.8)≥2)=0.896. The actual sample is drawn without replacement.
Original practice · fully worked
Original variant: both supervisors on a represented panel
A project group contains two supervisors and seven analysts. Three people are selected uniformly without replacement for a review panel. Given that the panel contains at least one supervisor, calculate the probability that it contains both supervisors.
A 0.0833
B 0.1429
C 0.4167
D 0.5833
E 0.8571
Variant answer in brief
Seven of the 84 possible panels contain both supervisors. Forty-nine panels contain at least one supervisor. Conditioning gives 7/49, or 1/7, approximately 0.1429. This is choice B.
Setup
Setup
Count all three-person panels and panels with no supervisor.
Nall=(39)=84,N0=(37)=35
Model
Model
Use the complement to count panels represented by at least one supervisor.
N≥1=84−35=49
Compute
Compute
Count panels with both supervisors and divide by the conditioning count.
N2=(22)(17)=7
Pr(H=2∣H≥1)=497=71=0.142857…
Answer
Answer
Given supervisor representation, both supervisors serve with probability approximately 0.1429.
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