This Exam P sample reference tests Conditional Probability. The conditional-probability ratio implies that the fire marginal is twice the flood marginal. The outside-versus-overlap relation then makes their marginal sum 0.80, so the flood probability is 0.80/3=0.2667 and choice A.
DThis reports the other marginal: f=2w=8/15=0.5333, rather than the flood marginal.
EThis reports the sum f+w=0.80 as if it were the probability of one policy; the sum also double-counts the intersection.
Original practice · fully worked
Original variant: reverse condition for sensor alerts
On a fleet-monitoring day, event H means at least one heat alert and event V means at least one vibration alert. Suppose P(H)=0.40, P(H union V)=0.65, and P(V given H)=0.25. Calculate P(H given V).
A 0.100
B 0.250
C 0.286
D 0.350
E 0.650
Variant answer in brief
The forward conditional probability gives an intersection of 0.10. Inclusion-exclusion then gives P(V)=0.35, so the reverse conditional probability is 0.10/0.35=2/7 and choice C.
Setup
Setup
Convert the supplied forward conditional probability into the joint alert probability.
P(H∩V)=P(V∣H)P(H)=0.25(0.40)=0.10
Model
Model
Use inclusion-exclusion to recover the vibration-alert marginal from the union, the heat marginal, and the intersection.
P(H∪V)=P(H)+P(V)−P(H∩V)
Compute
Compute
Solve for P(V), then condition the joint alert probability on V.
P(V)=0.65−0.40+0.10=0.35
P(H∣V)=P(V)P(H∩V)=0.350.10=72
Answer
Answer
The reverse conditional probability rounds to 0.286.
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.