This Exam P sample reference tests Conditional Probability. The probability of having neither procedure is the complement of 0.35, namely 0.65. Dividing this joint no-procedure probability by the 0.70 probability of no fillings gives 13/14, approximately 0.93, and choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.50 divides the probability of at least one procedure by the probability of no fillings. Those events are neither nested nor the requested pair.
BThe value 0.65 is the unconditional probability of having neither procedure. It has not been normalized by the no-fillings event.
CThe value 0.72 is approximately 0.65 divided by 0.90. That reverses the conditioning and calculates no fillings given no root canals.
DThe value 0.78 is approximately 0.70 divided by 0.90. It divides two marginals instead of using their joint probability.
Original practice · fully worked
Original variant: exactly one server alert
During a monitoring interval, a server has probability 0.30 of raising a capacity alert and probability 0.25 of raising a latency alert. The probability of raising neither alert is 0.55. Given that at least one alert is raised, calculate the probability that exactly one alert is raised.
A 0.100
B 0.350
C 0.450
D 0.550
E 0.778
Variant answer in brief
At least one alert occurs with probability 0.45. Inclusion–exclusion gives overlap 0.10, so exactly one alert occurs with probability 0.35. Dividing by the conditioning probability gives approximately 0.778 and choice E.
Setup
Setup
Complement the no-alert probability.
Pr(A∪B)=1−0.55=0.45
Model
Model
Use inclusion–exclusion to recover the overlap.
Pr(A∩B)=0.30+0.25−0.45=0.10
Compute
Compute
Remove the overlap twice from the marginal sum, then condition on the union.
Pr(exactly one)=0.30+0.25−2(0.10)=0.35
Pr(exactly one∣A∪B)=0.450.35=97
Answer
Answer
Given at least one alert, exactly one is raised with probability approximately 0.778.
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