This Exam P sample reference tests Geometric Distribution. This is a geometric waiting-time probability. One experiment succeeds on either of two all-matching outcomes, so its success probability is 1/4; two failures followed by a success have probability (3/4)²(1/4)=9/64, or 0.140625 and choice C.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.047 is 3/64=(3/4)(1/4)². It uses one failure followed by two successes instead of two failures followed by the first success.
BThe value 0.070 is 9/128, obtained by retaining only one of the two terminal all-matching success configurations while still using the combined failure probability.
DThe value 0.188 is 3/16=(3/4)(1/4), the probability that the first success occurs on experiment two.
EThe value 0.422 is 27/64=(3/4)³, the probability that all first three experiments fail.
Original practice · fully worked
Original variant: which independent uploader finishes first
Uploaders A and B make one independent transmission attempt per round until their own first success. Each A attempt succeeds with probability 0.25, and each B attempt succeeds with probability 0.50. Calculate the probability that uploader A succeeds in a strictly earlier round than uploader B.
A 0.125
B 0.200
C 0.250
D 0.333
E 0.800
Variant answer in brief
If A first succeeds in round k while B remains unsuccessful through that round, the probability is 0.25(0.50)(0.75 × 0.50)⁽ᵏ⁻¹⁾. Summing the geometric series gives 0.125/(1-0.375)=0.20, so choice B.
Setup
Setup
Let T_A and T_B be the two independent geometric completion rounds.
pA=0.25,qA=0.75,pB=0.50,qB=0.50
Model
Model
For A to win in round k, both uploaders must fail each earlier round, then A succeeds while B fails in round k.
Pr(TA=k,TB>k)=pAqB(qAqB)k−1
Compute
Compute
Sum over every possible winning round.
Pr(TA<TB)=k=1∑∞0.125(0.375)k−1
=1−0.3750.125=0.20
Answer
Answer
Uploader A finishes strictly first with probability 0.200.
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.