This Exam P sample reference tests Expected Value. Death in year k requires survival through the preceding k-1 years, so its probability is 0.99⁽ᵏ⁻¹⁾ times 0.01. Weighting the five possible benefits by those probabilities gives 740.0747, which corresponds to choice B.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis is approximately 750(0.99)⁸=692.06: it first ignores the year-specific survival weights and then applies an extraneous eight-period survival factor.
CThis is 0.01 times the unweighted sum of the five benefits, 0.01(25000+20000+15000+10000+5000)=750. It omits survival to later years.
DThis is near the level-benefit shortcut 20000[1-(0.99)⁵]=980.20. Replacing the declining schedule by a roughly 20,000 level payment overstates the expectation.
EThis uses the maximum 25,000 benefit whenever death occurs during the term: 25000[1-(0.99)⁵]=1225.25. Later deaths receive smaller benefits.
Original practice · fully worked
Original variant: checkpoint rewards accumulated before failure
A research drone attempts three navigation checkpoints in sequence. Conditional on reaching a checkpoint, its probabilities of completing checkpoints 1, 2, and 3 are 0.80, 0.75, and 0.50. It earns 100 units immediately after each completed checkpoint and an additional 300 units if it completes all three. Calculate the expected total reward.
A 90
B 170
C 180
D 260
E 295
Variant answer in brief
The cumulative checkpoint-completion probabilities are 0.80, 0.60, and 0.30. The three checkpoint rewards contribute 170 in expectation and the completion bonus contributes 90, for a total of 260 and choice D.
Setup
Setup
Convert the conditional success rates into cumulative probabilities of completing each checkpoint.
s1=0.80,s2=0.80(0.75)=0.60
s3=0.80(0.75)(0.50)=0.30
Model
Model
Use one indicator for each completed checkpoint. The bonus uses the third indicator because checkpoint 3 can be completed only after the first two.
R=100(I1+I2+I3)+300I3
Compute
Compute
Apply linearity of expectation to the accumulated checkpoint rewards and the terminal bonus.
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