This Exam P sample reference tests Exponential Distribution. In thousands, a payment above 5 requires the loss excess over the 0.5 deductible to exceed 5/0.8=6.25. The loss must therefore exceed 6.75, whose exponential survival probability is exp(-6.75/6)=0.32465, so choice C is correct.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis reflects a severe unit or threshold conversion error; the payment condition should produce a loss threshold of 6.75 in the model's thousand-unit scale.
BThis models the payment as 0.8L-0.5, applying coinsurance before subtracting the deductible, rather than 0.8(L-0.5).
DThis is the complement of the correct exponential tail and therefore represents the probability that the payment does not exceed the threshold.
EThis is the complement of the value generated by the incorrect transformation 0.8L-0.5, combining both a payment-model error and a reversed tail.
Original practice · fully worked
Original variant: conditional repair-payment percentile
A repair cost X follows an exponential distribution with mean 4 thousand dollars. A service plan pays 60% of the amount above a 1-thousand-dollar deductible. Given that the plan makes a positive payment, calculate the 90th percentile of the payment, in thousands.
A 2.400
B 4.926
C 5.526
D 6.126
E 9.210
Variant answer in brief
Given a positive payment, exponential memorylessness makes the excess over the deductible exponential with mean 4. Multiplying by 0.60 gives a conditional payment with mean 2.4. Its 90th percentile is 2.4ln(10)=5.526, choice C.
Setup
Setup
Let R be the service-plan payment and condition on the event that the repair cost clears the deductible.
R=0.6(X−1)+
R>0⟺X>1
Model
Model
Memorylessness makes the deductible excess exponential with the original mean; multiplying by 0.6 rescales that mean.
X−1∣(X>1)∼Exponential(mean 4)
R∣(R>0)∼Exponential(mean 2.4)
Compute
Compute
At the 90th percentile q, the conditional survival probability is 0.10.
e−q/2.4=0.10
q=2.4ln(10)=5.526204223…
Answer
Answer
The conditional 90th percentile of the payment is approximately 5.526 thousand dollars.
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