This Exam P sample reference tests Sampling Without Replacement. A match must occur by draw five, and reaching that maximum requires the first four draws to have four different colors. The sequential probability is (6/7)(4/6)(2/5)=8/35=0.2286, choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.0006 is approximately 1/(8·7·6·5), the probability of one particular ordered sequence of four individual socks. Many sequences have four distinct colors.
BThe value 0.0095 is the probability of one prescribed order of the four colors, (2/8)(2/7)(2/6)(2/5). There are 4! possible color orders.
CThe value 0.0417 is 1/24, the share of one color order among the 24 orders conditional on four distinct colors. It is not the unconditional event probability.
DThe value 0.1429 is 1/7, the probability that the second draw matches the first. That produces the minimum stopping time, not the maximum.
Original practice · fully worked
Original variant: first marked position in a random ordering
A sealed rack contains five green tags and seven amber tags. A technician exposes tags one at a time in a uniformly random order and stops upon seeing the first green tag. If T includes the stopping exposure, determine E[T].
A 1.167
B 2.000
C 2.167
D 2.400
E 8.000
Variant answer in brief
The five green positions form a uniform five-subset of the twelve positions. The expected minimum selected position is (12+1)/(5+1)=13/6=2.1667, choice C.
Setup
Setup
View the random exposure order as five marked green positions among twelve positions.
1≤T≤8
T=min{green positions}
Model
Model
For K marked positions chosen uniformly from N positions, the expected first marked position is (N+1)/(K+1).
E[T]=K+1N+1
Compute
Compute
Insert the twelve total positions and five green positions.
E[T]=5+112+1=613=2.1666667
Answer
Answer
The expected stopping exposure is approximately 2.167.
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