This Exam P sample reference tests Conditional Probability. First combine the event probability and its conditional response probability to get a one-year response chance of 0.0875. Independence across three years makes the at-least-once probability 1-(1-0.0875)³=0.240201, so choice C is correct.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis reports the single-year joint probability 0.25(0.35)=0.0875 and never accounts for three opportunities.
BThis computes exactly one response, 3(0.0875)(0.9125)²=0.218572, rather than at least one response.
DThis is 0.75³=0.421875, the probability that the triggering event is absent in all three years.
EThis is 1-0.75³=0.578125, the probability of at least one trigger; it omits the conditional response requirement.
Original practice · fully worked
Original variant: quality-audit escalation rate
A plant runs four independent quality shifts. On each shift, an audit is selected with probability 0.40. If selected, the audit logs an escalation with an unknown probability r. The probability of at least one logged escalation over the four shifts is 0.15065344. Calculate r.
A 0.040
B 0.100
C 0.151
D 0.377
E 0.960
Variant answer in brief
The probability of no escalation on every shift is 0.84934656=0.96⁴, so the one-shift escalation probability is 0.04. Since that joint probability is 0.40r, r=0.10 and choice B is correct.
Setup
Setup
Take the complement to obtain the probability that all four shifts finish without an escalation.
Pr(no escalation in four shifts)=1−0.15065344=0.84934656
Model
Model
If q is the one-shift escalation probability, independence gives a fourth power for four escalation-free shifts.
(1−q)4=0.84934656
Compute
Compute
Recover the one-shift joint probability, then divide by the audit-selection probability.
1−q=(0.84934656)1/4=0.96
q=0.04=0.40r
r=0.10
Answer
Answer
A selected audit escalates with probability 0.100.
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