This Exam P sample reference tests Combinatorial Probability. This is an at-least probability under sampling without replacement. Adding the hypergeometric terms for exactly three and exactly four target members gives 25/42=0.595238, which rounds to choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AA value of 0.10 is already below P(X=4)=15/126=0.1190, so it cannot represent the larger event X at least 3.
BThis is P(X=4)=15/126=0.1190 rounded to 0.12; it omits all samples with exactly three target members.
CA value near 0.14 adds very little to the exactly-four probability 0.1190 and therefore misses most of the exactly-three contribution 60/126=0.4762.
DThis is P(X=3)=60/126=0.4762 rounded to 0.48; it omits the valid exactly-four outcome.
Original practice · fully worked
Original variant: locating the second priority capsule
Seven capsules are placed in a uniformly random order. Four are marked priority and three are regular. Calculate the probability that the second priority capsule appears in position four.
A 48/245
B 9/35
C 12/35
D 4/7
E 26/35
Variant answer in brief
Exactly one of the first three positions must be priority, position four must be priority, and two of the final three positions must be priority. Counting priority-position sets gives 9/35, so choice B.
Setup
Setup
Describe an ordering by the four positions occupied by priority capsules. All four-position subsets are equally likely.
#{priority-position sets}=(47)=35
Model
Model
For the second priority capsule to occupy position four, choose one priority position among the first three, fix position four, and choose two priority positions among the last three.
#{favorable sets}=(13)(23)=9
Compute
Compute
Divide favorable position sets by all possible position sets.
Pr(second priority at position 4)=359=0.2571428571…
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