Original practice · fully worked
Original variant: capped endurance reading
A battery endurance X has survival function 1 for 0≤x<200 and (200/x)² for x≥200. A test rig records the smaller of X and an upper display limit L. If the expected displayed reading is 360, determine L.
- A
360 - B
400 - C
500 - D
1000 - E
2000
Answer and calculation
The limited expectation is the survival integral through L, equal to 400-40000/L. Setting this expression to 360 gives L=1000, so choice D is correct.
Setup
Setup
Express a capped nonnegative expectation as an integral of the survival function up to the cap.
E[min(X,L)]=∫0LPr(X>x)dx Model
Model
Split the integral at the Pareto lower endpoint.
E[min(X,L)]=∫02001dx+∫200L(x200)2dx E[min(X,L)]=400−L40000 Compute
Compute
Equate the limited expectation to the observed display mean and solve for the upper limit.
400−L40000=360 L40000=40 Answer
Answer
The display limit is 1000 endurance units.
L=1000(D)