Independent solution

How to solve this Uniform Distribution question

Setup

Setup

Use the raw second moment formula for a uniform variable whose endpoints are a and 100.

E[X2]=a2+100a+10023=196003E[X^2]=\frac{a^2+100a+100^2}{3}=\frac{19600}{3}

Model

Model

Solve the resulting quadratic and retain the root that forms a valid interval below 100.

a2+100a9600=0a^2+100a-9600=0
(a60)(a+160)=0(a-60)(a+160)=0

Compute

Compute

The valid lower endpoint is 60, so the second model begins at 1.25 × 60. Move 80% of the way across that interval.

a=60,1.25a=75a=60,\qquad 1.25a=75
q0.80(Y)=75+0.80(10075)=95q_{0.80}(Y)=75+0.80(100-75)=95

Answer

Answer

The requested percentile is 95.

95(E)\boxed{95\quad\text{(E)}}