This Exam P sample reference tests Geometric Distribution. This is a geometric stopping-time calculation. A round continues with probability (1/4)³+(3/4)³=7/16, so exactly five rounds requires four continuations followed by a stop: (7/16)⁴(9/16)=0.020608, making choice C correct.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis treats continuation as a prescribed rare matching pattern rather than allowing both possible all-matching outcomes.
BThis over-multiplies the rare all-first-outcome event instead of using the full one-round continuation probability 7/16.
DThis is close to an off-by-one calculation with only three continuation rounds before the stopping round.
EThis is close to r²(1-r), the probability of stopping on round three rather than round five.
Original practice · fully worked
Original variant: two-state incident response
An incident starts at level 0. During the first response cycle it is resolved with probability 0.3 or advances to level 1 with probability 0.7. Once at level 1, each later cycle resolves it with probability 0.6 or leaves it at level 1 with probability 0.4. Calculate the probability that the incident is resolved exactly on cycle 4.
A 0.0288
B 0.0672
C 0.0960
D 0.1680
E 0.2800
Variant answer in brief
Resolution on cycle 4 requires advancement on cycle 1, two further unresolved level-1 cycles, and resolution on cycle 4. The probability is 0.7(0.4)²(0.6)=0.0672, so choice B is correct.
Setup
Setup
Identify the unique state path that permits resolution on the fourth cycle.
0⟶1⟶1⟶1⟶resolved
Model
Model
The first transition has probability 0.7, the next two cycles must remain unresolved, and the final cycle must resolve.
Pr(T=4)=0.7(0.4)(0.4)(0.6)
Compute
Compute
Multiply the transition probabilities along the required path.
Pr(T=4)=0.7(0.4)2(0.6)=0.0672
Answer
Answer
The incident resolves on cycle 4 with probability 0.0672.
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