This Exam P sample reference tests Set Probability. Complementing the two supplied percentages gives probabilities 0.60 for the three-year event and 0.30 for combined coverage. The availability restrictions make these events disjoint, so their union has probability 0.90 and choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis is the probability of the impossible intersection T cap B, but the question asks for the union rather than the intersection.
BThe value 0.18 is 0.60(0.30), obtained by assuming independence and calculating an intersection; the table instead establishes mutual exclusivity.
CThe value 0.28 is (1-0.60)(1-0.30), the probability of neither event under an unjustified independence assumption.
DThe value 0.72 is 0.60+0.30-0.60(0.30), which applies the union formula after incorrectly assuming that the two events are independent.
Original practice · fully worked
Original variant: observatory instrument scheduling
Every observation block at a research observatory uses an optical instrument, a radio instrument, or both. Sixty-five percent of blocks use exactly one instrument. Also, 52 percent occur during daylight or twilight. A safety rule prohibits blocks using both instruments from being scheduled at night. Calculate the probability that a randomly selected block is at night or uses both instruments.
A 0.000
B 0.168
C 0.338
D 0.662
E 0.830
Variant answer in brief
Night blocks have probability 0.48 and dual-instrument blocks have probability 0.35. The safety rule makes those events disjoint, so their union is 0.83 and choice E.
Setup
Setup
Let N denote a night block and D a block using both instruments. Convert the supplied complementary percentages.
Pr(N)=1−0.52=0.48
Pr(D)=1−0.65=0.35
Model
Model
The scheduling rule excludes every dual-instrument block from the night period.
Pr(N∩D)=0
Compute
Compute
Add the probabilities of the two disjoint events.
Pr(N∪D)=0.48+0.35=0.83
Answer
Answer
The requested union contains 83 percent of the scheduled blocks.
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.