This Exam P sample reference tests Independence. The emergency-charge marginal is 0.75. Substituting independence into the union formula yields operating-charge probability 0.40, choice D.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.10 is the residual after rearranging the union equation. It is 0.25p, not p itself.
BSubstituting p=0.20 gives a union probability of 0.80 rather than the supplied 0.85.
CThe value 0.25 is the coefficient of p after the union equation is simplified; it is not the solution for p.
Original practice · fully worked
Original variant: recover a marginal from an independent union
Two independent diagnostic flags A and B are considered. Flag B is absent with probability 0.40, and at least one flag appears with probability 0.88. Find P(A).
A 0.30
B 0.60
C 0.70
D 0.80
E 0.88
Variant answer in brief
Independence gives P(A absent and B absent)=0.12. Dividing by P(B absent)=0.40 gives P(A absent)=0.30, so P(A)=0.70, choice C.
Setup
Setup
Complement the at-least-one event to obtain the probability that both flags are absent.
Pr(Ac∩Bc)=1−0.88=0.12
Model
Model
Independence makes the joint absence probability the product of the two marginal absence probabilities.
0.12=Pr(Ac)Pr(Bc)=0.40Pr(Ac)
Compute
Compute
Divide 0.12 by Pr(B absent)=0.40 to get Pr(A absent)=0.30, then take the complement to obtain Pr(A)=0.70.
Pr(Ac)=0.30,Pr(A)=0.70
Answer
Answer
Flag A appears with probability 0.70, which is choice C.
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