This Exam P sample reference tests Inclusion-Exclusion. Three-set inclusion-exclusion gives 450 policies having at least one listed home feature. Subtracting this union from the 1000-policy portfolio leaves 550 with none, choice D.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 310 is 1000 minus all seven supplied counts after adding the pair and triple intersections instead of using alternating inclusion-exclusion signs.
BThe value 450 is the number with at least one listed feature. The requested count is its complement in the 1000-policy portfolio.
CThe value 530 results from converting the pair intersections to pair-only counts 30, 20, and 40 and then subtracting them without the remaining triple-overlap correction.
EThe value 570 results from subtracting the triple intersection in the union formula instead of adding it, producing an understated union of 430.
Original practice · fully worked
Original variant: exactly one independent sensor trigger
Three independent sensors trigger during a test with probabilities 0.25, 0.40, and 0.10. Calculate the probability that exactly one sensor triggers.
A 0.010
B 0.145
C 0.405
D 0.450
E 0.595
Variant answer in brief
The three disjoint one-trigger cases have probabilities 0.135, 0.270, and 0.045. Their sum is 0.450, choice D.
Setup
Setup
List the three mutually exclusive ways exactly one sensor can trigger.
ABˉCˉ,AˉBCˉ,AˉBˉC
Model
Model
Use independence to multiply each trigger probability by the other two non-trigger probabilities.
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