This Exam P sample reference tests Binomial Distribution. The twenty-year count is binomial. Summing its probabilities for zero, one, and two events gives 0.92452, choice E.
Original variant: exactly one outage conditional on at most one
During each of fifteen independent service periods, an outage occurs with probability 0.08 and at most one outage can occur per period. Given that the total number of outages is no more than one, find the probability that it is exactly one.
A 0.4340
B 0.5000
C 0.5400
D 0.5660
E 0.6597
Variant answer in brief
The ratio of the one-outage mass to the zero-outage mass is 15(0.08)/0.92. Normalizing those two masses gives 0.5660, choice D.
Setup
Setup
The total outage count across 15 independent periods is binomial with outage probability 0.08.
N∼Bin(15,0.08)
Model
Model
Under the condition of at most one outage, only the zero- and one-outage masses remain. Their ratio simplifies before normalization.
Pr(N=0)Pr(N=1)=0.9215(0.08)=2330
Compute
Compute
Normalizing the two admissible masses gives an exactly-one probability of approximately 0.566038.
Pr(N=1∣N≤1)=30+2330=0.566038
Answer
Answer
The conditional probability of exactly one outage is approximately 0.5660, selecting choice D.
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