This Exam P sample reference tests Exponential Distribution. The capped payment equals its maximum whenever the underlying loss reaches or exceeds 3000. An exponential loss with mean 2000 has survival probability e raised to minus 1.5 at that threshold, approximately 0.223 and choice B.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis uses the fact that a continuous loss equals exactly 3000 with probability zero but overlooks the probability mass created at the cap by every larger loss.
CThis is approximately 0.487, the complement obtained after reversing the mean and threshold in the exponential exponent.
DThis reverses the mean and threshold in the exponential exponent, producing a survival value of approximately 0.513.
EThis is the probability that the loss remains below 3000, so the payment is less than its maximum.
Original practice · fully worked
Original variant: capped total from two gauges
Two independent runoff gauges each report an amount uniformly distributed from 0 to 10 units. A reimbursement equals their combined reading, but is capped at 12 units. Calculate the probability that the reimbursement equals exactly 12 units.
A 0.000
B 0.080
C 0.320
D 0.500
E 0.680
Variant answer in brief
The cap is reached when the two readings sum to at least 12. In the 10-by-10 sample square, that event is an upper triangle with legs of length 8 and area 32, giving probability 0.32 and choice C.
Setup
Setup
Translate the exact capped value into an event for the uncapped sum.
R=min(U+V,12),{R=12}={U+V≥12}
Model
Model
The independent readings are uniform over a square of area 100.
0≤U≤10,0≤V≤10
Compute
Compute
The favorable upper triangle has legs from 2 to 10 on each axis.
Pr(U+V≥12)=10021(8)(8)=0.32
Answer
Answer
The reimbursement reaches its cap with probability 0.320.
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