This Exam P sample reference tests Exponential Distribution. The payment median is positive because the zero-payment probability is below one half. The corresponding loss threshold is the loss median 4 ln(2), so subtracting the deductible gives 1.7726, choice A.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
BThe value 2.08 is approximately (4-1)ln(2). It incorrectly reduces the exponential mean by the deductible before calculating a median.
CThe value 2.12 is approximately 4exp(-1/4)-1. It first computes the mean payment and then subtracts the deductible a second time.
DThe value 2.77 is the median loss 4ln(2). It does not subtract the deductible from the positive loss threshold.
EThe value 3.12 is the mean payment E[(X-1)+]=4exp(-1/4), not its median.
Original practice · fully worked
Original variant: expected recording time after logger activation
A battery lifetime X is exponentially distributed with mean 4 hours. A diagnostic logger activates if the battery remains operating after hour 2 and then records for at most 3 additional hours. Given that the logger activates, calculate its expected recording time.
A 0.6065
B 1.2801
C 2.1105
D 2.4261
E 3.0000
Variant answer in brief
Conditional on reaching hour 2, exponential memorylessness makes the remaining lifetime exponential with mean 4. Recording at most 3 additional hours gives expectation 4(1-exp(−3/4))=2.1105, choice C.
Setup
Setup
Write the capped recording duration and condition on logger activation.
R=min((X−2)+,3)
{logger activates}={X>2}
Model
Model
Memorylessness makes the remaining lifetime a fresh exponential variable Z with mean 4.
Z=(X−2)∣X>2∼Exponential(mean 4)
R∣X>2=min(Z,3)
Compute
Compute
Use the survival-integral formula for the limited mean.
E[R∣X>2]=∫03Pr(Z>t)dt
=∫03e−t/4dt=4(1−e−3/4)=2.1105338
Answer
Answer
Given activation, the expected recording time is approximately 2.1105 hours.
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.