Independent solution

How to solve this Conditional Probability question

Setup

Setup

Integrate the density from x to the upper endpoint 20 to obtain the survival function.

S(x)=x200.005(20t)dt=0.0025(20x)2S(x)=\int_x^{20}0.005(20-t)\,dt=0.0025(20-x)^2

Model

Model

Because X>16 is contained in X>8, the conditional probability is the ratio of the two survival probabilities.

Pr(X>16X>8)=S(16)S(8)\Pr(X>16\mid X>8)=\frac{S(16)}{S(8)}

Compute

Compute

The common factor cancels, leaving the squared distance ratio (4/12)²=1/9.

0.0025(4)20.0025(12)2=19\frac{0.0025(4)^2}{0.0025(12)^2}=\frac19

Answer

Answer

Therefore Pr(X>16 given X>8)=1/9, corresponding to choice B.

19(B)\boxed{\frac19\quad\text{(B)}}