This Exam P sample reference tests Binomial Distribution. The number of covered occurrences is binomial with five trials and probability 0.5. Capping the count at three gives an expected payment of 30(73/32) = 68.4375, so the nearest listed value is D.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis treats the three-payment limit as three ordinary expected trials, giving 3(30)(0.5), instead of applying the limit to the five-period count.
BThis undercounts the upper tail as though only two payments were available and then rounds the result.
CThis is obtained by omitting the five-occurrence case from the capped upper tail, producing 65.625 before rounding.
EThis is the uncapped expectation 30(5)(0.5), so it ignores that counts above three receive no additional payment.
Original practice · fully worked
Original variant: fabrication safety triggers
A fabrication line completes seven independent production runs. Its safety relay is triggered on any run with probability 0.25. A reliability rebate ignores the first two triggers, then pays 50 for each additional trigger, subject to at most three paid triggers. Calculate the expected rebate.
A 0.000
B 12.180
C 16.351
D 16.422
E 83.258
Variant answer in brief
For a binomial trigger count N, the number of paid triggers is min((N-2)+,3). Its expectation is the sum of the three binomial tails beginning at 3, 4, and 5, which equals 0.3270263672. Multiplication by 50 gives 16.35131836, so choice C is correct.
Setup
Setup
Let N count relay triggers over the seven independent runs and translate the two-trigger threshold and three-payment ceiling into a payment variable.
N∼Binomial(7,0.25)
R=50min((N−2)+,3)
Model
Model
Apply the tail-sum identity to the paid-trigger count, whose possible values are zero through three.
E[min((N−2)+,3)]
=Pr(N≥3)+Pr(N≥4)+Pr(N≥5)
Compute
Compute
Evaluate the three required binomial tails and multiply their sum by the rebate per paid trigger.
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