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
Answer and calculation
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.
Pr(N≥3)+Pr(N≥4)+Pr(N≥5) =0.2435913086+0.0705566406+0.0128784180 =0.3270263672 E[R]=50(0.3270263672)=16.35131836 Answer
Answer
The expected reliability rebate rounds to 16.351.
16.351(C)