Original practice · fully worked
Original variant: lower quartile of an inspection minimum
A coating score X has density f(x)=2(1-x) for 0<x<1. Four scores are sampled independently, and M is their minimum. Calculate the 25th percentile of M.
- A
0.0353 - B
0.1340 - C
0.1591 - D
0.4588 - E
0.9647
Answer and calculation
One score has survival function (1-x)², so the four-score minimum has survival function (1-m)⁸. Setting its CDF to 0.25 gives m=1-0.75⁽¹⁄⁸⁾=0.035321, so choice A is correct.
Setup
Setup
Integrate the density and write the survival function of one score.
FX(x)=1−(1−x)2 SX(x)=(1−x)2 Model
Model
The minimum exceeds m only if all four independent scores exceed m.
Pr(M>m)=SX(m)4=(1−m)8 FM(m)=1−(1−m)8 Compute
Compute
Set the minimum's CDF equal to one quarter and solve.
1−(1−m)8=0.25 m=1−0.751/8=0.0353213700 Answer
Answer
The lower quartile of the minimum rounds to 0.0353.
0.0353(A)