This Exam P sample reference tests Hypergeometric Distribution. Choose one of two items in the first category, two of four in the second, and three of seven in the third. Dividing the resulting 420 favorable subsets by C(13,6)=1716 gives 0.24476, choice D.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.025 is near C(2,1)C(4,0)C(7,5)/C(13,6)=0.02448. That composition replaces the required two homeowners claims with two additional auto claims.
BA value near 0.107 results from undercounting category arrangements in an ordered-draw approach. There are 6!/(1!2!3!)=60 placements of the category pattern; the unordered combination count already includes them correctly.
CA value near 0.153 can arise from a with-replacement category approximation. The six claims are selected without replacement from finite category counts, so the multivariate hypergeometric count is required.
EThe value 0.643 includes many mixed-category samples beyond the one-two-three composition. The exact favorable count is 420 of the 1716 equally likely subsets.
Original practice · fully worked
Original variant: sample containing exactly two represented categories
A cabinet contains three red cartridges, four blue cartridges, and five green cartridges. Four cartridges are sampled without replacement. What proportion of such samples contain cartridges in just two colors?
A 0.0121
B 0.4424
C 0.4667
D 0.5455
E 0.9879
Variant answer in brief
Count four-subsets within each color pair and remove monochromatic subsets. The three pair contributions are 34, 65, and 120, so 219 of 495 samples contain exactly two colors, giving 0.4424, choice B.
Setup
Setup
Count all four-cartridge subsets.
#{all samples}=(412)=495
Model
Model
For each pair of colors, count subsets drawn from that pair and exclude one-color subsets.
NRB=(47)−(44)=34
NRG=(48)−(45)=65
NBG=(49)−(44)−(45)=120
Compute
Compute
Add the disjoint exact-two-color counts and divide by all samples.
Nexactly two=34+65+120=219
Pr(exactly two colors)=495219=16573=0.4424242
Answer
Answer
Exactly two colors are represented with probability about 0.4424.
The 3108-page Probability Proof Manual reorganizes 718 verified Exam P solutions by syllabus skill and adds formula proofs, error patterns, and original worked practice.