This Exam P sample reference tests Continuous Random Variables. Integrate the density shape and divide by its total integral. Setting the resulting CDF equal to one-half gives m=√(√(26)-1)=2.0246, which matches choice A.
How to solve this Continuous Random Variables question
Setup
Setup
The unknown proportionality constant cancels when the partial integral is divided by the full-support integral.
∫1+x2xdx=21ln(1+x2)
Model
Model
Form the CDF on the support from the ratio of accumulated density shape to total density shape.
F(x)=21ln(26)21ln(1+x2)=ln(26)ln(1+x2),0<x<5
Compute
Compute
A median divides the probability mass into equal halves.
ln(26)ln(1+m2)=21
1+m2=26
m=26−1=2.0246035…
Answer
Answer
The closest listed value is 2.02.
m≈2.02(A)
Test the method on unseen problems
Probability Univariate Distributions Practice Workbook
30 original univariate random variables problems with complete solutions and wrong-choice analysis. Written for Exam P, not copied from the SOA set. One-time $19 PDF.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
BDropping the 1 after exponentiating gives m=26¹⁄⁴=2.2581, which rounds to 2.26.
CThis uses the midpoint of the support. A median need not be the midpoint when density is not symmetric.
DDirect substitution gives F(2.74) about 0.657, so this value leaves substantially more than half the mass below it.
EDirect substitution gives F(2.98) about 0.703, making this an even higher quantile rather than the median.
Original practice · fully worked
Original variant: optical response level
A laboratory sensor reports a response level Z between 0 and 3. Its probability density is proportional to 1+z on that interval and is zero elsewhere. Calculate the 80th percentile of Z.
A 1.646
B 2.000
C 2.400
D 2.606
E 3.606
Answer and calculation
The accumulated density shape is z+z²⁄² and its total is 7.5. Setting the ratio equal to 0.80 gives z=-1+√(13)=2.6056, so choice D is correct.
Setup
Setup
Integrate the density shape over the entire support.
∫03(1+z)dz=3+232=7.5
Model
Model
Express the CDF as a ratio so the proportionality constant cancels.
F(z)=7.5z+z2/2,0<z<3
Compute
Compute
Set the CDF equal to 0.80 and solve the admissible quadratic root.