This Exam P sample reference tests Survival Functions. A unit survives through the second-year endpoint with probability 2⁻¹⁄⁴. Independence makes the three-unit no-failure probability 2⁻³⁄⁴, so the expected 1000 payment is 1000(1-2⁻³⁄⁴)=405.396 and choice C.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis pays only when all three units fail: with q=1-2⁻¹⁄⁴, the calculation 1000q³=4.028 rounds to 4. The contract triggers on at least one failure.
BThis considers only one specified unit, giving 1000q=159.104. There are three independent opportunities for the covered event.
DThis multiplies 1000 by the probability of no covered failure, 2⁻³⁄⁴=0.594604, rather than by its complement.
EThis uses 1-q³=0.995972, the complement of all three units failing. The needed complement is of zero failures, not of three failures.
Original practice · fully worked
Original variant: infer a batch-discard probability
Four sensors are calibrated independently and have a common probability p of producing an acceptable reading. The probability that all four readings are acceptable is 0.4096. A batch is discarded when at least two readings are unacceptable. Calculate the discard probability.
A 0.0016
B 0.1808
C 0.4096
D 0.5904
E 0.8192
Variant answer in brief
The all-acceptable probability gives p⁴=0.4096 and hence p=0.80. For K, the number unacceptable, P(K≥2)=1-P(K=0)-P(K=1)=1-0.4096-0.4096=0.1808, so choice B.
Setup
Setup
Recover the one-sensor acceptable probability from the four-sensor joint probability.
p4=0.4096
p=0.80,1−p=0.20
Model
Model
Let K count unacceptable readings. The discard event is K at least two, whose complement contains K=0 and K=1.
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