This Exam P sample reference tests Conditional Probability. The under-25 group has size 505. Its female portion is 0.57(414)=235.98, leaving 269.02 males, so the requested conditional probability is 269.02/505=0.5327, choice B.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.47 is the female share within the under-25 group, approximately 235.98/505. It is the complement of the requested conditional probability.
CThe value 0.54 is the unconditional male proportion in the entire portfolio. Conditioning on age changes that proportion.
DThe value 0.55 is approximately 269.02/486, which is the under-25 proportion among males, P(under 25|male), with the conditioning direction reversed.
EThe value 0.56 is the overall under-25 proportion, 505/900. It does not identify sex within that age group.
Original practice · fully worked
Original variant: supplier share among accepted components
A component inventory comes 40% from supplier A, 35% from supplier B, and 25% from supplier C. Defect rates are 2% for A and 4% for B, while the inventory-wide defect rate is 3.25%. A nondefective component is selected. Determine the probability that it came from supplier C.
A 0.2200
B 0.2395
C 0.2475
D 0.2500
E 0.9580
Variant answer in brief
The mixture defect rate implies supplier C has defect rate 0.042. Its nondefective mass is 0.25(0.958)=0.2395, and dividing by the total nondefective mass 0.9675 gives 0.2475, choice C.
Setup
Setup
Let q be the unknown defect probability for supplier C.
0.0325=0.40(0.02)+0.35(0.04)+0.25q
Model
Model
Solve the mixture equation and complement the resulting supplier-specific defect rate.
q=0.042,Pr(N∣C)=1−q=0.958
Compute
Compute
Apply Bayes' rule to the nondefective selection.
Pr(C∩N)=0.25(0.958)=0.2395
Pr(N)=1−0.0325=0.9675
Pr(C∣N)=0.96750.2395=0.2475452
Answer
Answer
About 24.75% of accepted components come from supplier C.
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