This Exam P sample reference tests Exponential Distribution. For a normal or continuous uniform distribution, symmetry makes the median equal to the mean. For an exponential distribution, the median is the mean multiplied by the constant natural logarithm of two. In every case, doubling the mean therefore doubles the median, so choice A is correct.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
BThis excludes the normal family. A normal distribution is symmetric about its mean, so its median equals its mean and scales with it.
CThis excludes the continuous uniform family. Its midpoint is both mean and median, so the two quantities change together.
DThis excludes the exponential family. Its median is the natural logarithm of two times its mean, a fixed proportional relationship.
EAt least the normal and continuous uniform families qualify by symmetry, and the exponential family qualifies by its fixed median-to-mean ratio.
Original practice · fully worked
Original variant: median resale credit from wear grades
A machine's wear grade G is 1, 4, 7, or 10 with probabilities 0.20, 0.35, 0.25, and 0.20, respectively. Its resale credit is C=100/(G+1) dollars. Calculate the median resale credit.
A 4.000
B 9.091
C 15.748
D 20.000
E 25.000
Variant answer in brief
The cumulative probability of the wear grade first exceeds one half at G=4, so 4 is its unique median. The decreasing credit rule carries that median to 100/(4+1)=20 dollars, and direct ordering of the credit masses confirms choice D.
Setup
Setup
Locate where the cumulative wear-grade probability first reaches one half.
Pr(G≤1)=0.20,Pr(G≤4)=0.20+0.35=0.55
mG=4
Model
Model
The credit is a strictly decreasing function of grade. A unique median maps to the corresponding transformed value even though the ordering reverses.
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