This Exam P sample reference tests Law of Total Probability. Class A contributes 0.008 to the hospitalization probability and Class B contributes 0.003. Thus Class A accounts for eight elevenths of all hospitalizations, approximately 0.727, and choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.011 is the overall hospitalization probability, obtained by adding the two class contributions. It is the conditioning denominator.
BThe value 0.020 is the hospitalization rate within Class A. The question reverses the conditioning.
CWeighting the no-hospitalization rates instead gives 0.392 divided by 0.989, approximately 0.396. That conditions on avoiding hospitalization rather than on hospitalization.
DThe value 0.400 is the unconditional Class A proportion. Hospitalization changes the class mix because the two conditional rates differ.
Original practice · fully worked
Original variant: predict a second roll from partial evidence
A simulation selects a fair six-sided die with probability two-thirds and a fair eight-sided die otherwise. The selected die is rolled twice. The first roll is greater than four. Given this result, calculate the probability that the second roll is also greater than four.
A 0.333
B 0.389
C 0.405
D 0.429
E 0.500
Variant answer in brief
The first result updates the die probabilities to four-sevenths for the six-sided die and three-sevenths for the eight-sided die. Averaging their chances of another result above four gives 17/42, approximately 0.405, and choice C.
Setup
Setup
Let E denote a roll greater than four. Calculate its likelihood under each possible die.
Pr(E∣D6)=62=31
Pr(E∣D8)=84=21
Model
Model
Use the first roll to update the probability assigned to each die.
Pr(E)=3231+3121=187
Pr(D6∣E)=74,Pr(D8∣E)=73
Compute
Compute
Conditional on the selected die, the two rolls are independent. Average the second-roll likelihoods over the updated die probabilities.
Pr(E2∣E1)=7431+7321
=4217=0.4047619048…
Answer
Answer
The second roll exceeds four with conditional probability approximately 0.405.
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