This Exam P sample reference tests Continuous Random Variables. The first density segment contains probability 0.40, so the median lies in the next segment. Accumulating another 0.10 at density 0.30 requires one-third of a year beyond 0.5, giving a median of 5/6 = 0.833333 year and choice B.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AAt 0.39 year the cumulative probability is only 0.80(0.39) = 0.312, so this value has not accumulated the required probability 0.50.
CAt 0.94 year the cumulative probability is 0.40 + 0.30(0.44) = 0.532; continuing too far into the second segment overshoots the median.
DThe value 1.38 is the rounded mean: direct integration gives E[X] = 1.375. A skewed distribution need not have its mean equal to its median.
EThe point 1.50 is a density breakpoint, but F(1.50) = 0.40 + 0.30(1.00) = 0.70, not 0.50.
Original practice · fully worked
Original variant: greenhouse sensor drift
The time R, in hundreds of operating hours, until a greenhouse sensor requires recalibration has density 0.30 for 0 ≤ R ≤ 1, density 0.20 for 1 < R ≤ 3, density 0.10 for 3 < R ≤ 6, and zero elsewhere. Calculate the median recalibration time.
A 1.2
B 1.6
C 2.0
D 2.4
E 2.8
Variant answer in brief
The first interval contributes probability 0.30. Accumulating the remaining 0.20 at density 0.20 takes one more unit, so the median is 2.0 hundreds of hours and choice C.
Setup
Setup
Confirm that the stated pieces form a probability density and identify the cumulative mass at each breakpoint.
0.30(1)+0.20(2)+0.10(3)=0.30+0.40+0.30=1
Model
Model
The first interval contains 0.30 probability, so the 50th percentile occurs in the second interval.
F(r)=0.30+0.20(r−1),1<r≤3
Compute
Compute
Solve the median equation inside that interval.
0.30+0.20(r−1)=0.50
r−1=1,r=2
Answer
Answer
The median is 2.0 hundreds of operating hours, or 200 hours.
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