This Exam P sample reference tests Independence. The husband's two-year survival probability is 0.90. Independence makes the joint survival probability the product of the two marginal survival probabilities, so the wife's survival probability is 7/9 and her death probability is 2/9, approximately 0.222. This is choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AA death probability of 0.100 simply copies the husband's rate. It would produce joint survival of 0.81, not the stated 0.70.
BA death probability of 0.118 would leave wife survival of 0.882 and hence joint survival of about 0.794. That fails the supplied joint probability.
CA death probability of 0.143 is the ratio of 0.10 to 0.70. It would imply joint survival of about 0.771 rather than 0.70.
DA death probability of 0.200 treats the two death events as though their probabilities could be removed directly from one. It implies joint survival of 0.72.
Original practice · fully worked
Original variant: simultaneous backup-link failures
Two backup communication links fail independently during a stress test. Their failure probabilities are 0.08 and 0.15. Given that at least one link fails, calculate the probability that both links fail.
A 0.012
B 0.055
C 0.080
D 0.150
E 0.218
Variant answer in brief
Independence gives a double-failure probability of 0.012, while inclusion–exclusion gives 0.218 for at least one failure. Their ratio is approximately 0.055, choice B.
Setup
Setup
Multiply the marginal failure probabilities to obtain the double-failure probability.
Pr(A∩B)=(0.08)(0.15)=0.012
Model
Model
Use inclusion–exclusion for the event that at least one link fails.
Pr(A∪B)=0.08+0.15−0.012=0.218
Compute
Compute
Condition the double-failure event on the union of the two failure events.
Pr(A∩B∣A∪B)=0.2180.012=0.05504587…
Answer
Answer
Given at least one failure, the probability that both links fail is approximately 0.055.
The 3108-page Probability Proof Manual reorganizes 718 verified Exam P solutions by syllabus skill and adds formula proofs, error patterns, and original worked practice.