Independent solution

How to solve this Continuous Random Variables question

Setup

Setup

Write the density as an unknown constant times the supplied kernel on its finite support.

f(x)=C(10+x)2,0<x<40f(x)=C(10+x)^{-2},\qquad 0<x<40

Model

Model

Normalize the density to determine the proportionality constant.

1=C040(10+x)2dx1=C\int_0^{40}(10+x)^{-2}\,dx
=C(110150)=C225=C\left(\frac1{10}-\frac1{50}\right)=C\frac2{25}
C=252C=\frac{25}{2}

Compute

Compute

Integrate the normalized density over the requested lower interval.

Pr(X<5)=25205(10+x)2dx\Pr(X<5)=\frac{25}{2}\int_0^5(10+x)^{-2}\,dx
=252(110115)=512=0.416666=\frac{25}{2}\left(\frac1{10}-\frac1{15}\right)=\frac5{12}=0.416666\ldots

Answer

Answer

The probability rounds to 0.42.

0.42(C)\boxed{0.42\quad\text{(C)}}