Independent solution

How to solve this Bayes' Theorem question

Setup

Setup

Let x denote the unknown smoking probability within the female group and form the two smoker contributions.

Pr(FS)=0.40x\Pr(F\cap S)=0.40x
Pr(MS)=0.60(0.20)=0.12\Pr(M\cap S)=0.60(0.20)=0.12

Model

Model

Express the supplied female share among smokers as a Bayes ratio.

0.20=Pr(FS)=0.40x0.40x+0.120.20=\Pr(F\mid S)=\frac{0.40x}{0.40x+0.12}

Compute

Compute

Solve the resulting linear equation for the within-group rate.

0.20(0.40x+0.12)=0.40x0.20(0.40x+0.12)=0.40x
0.08x+0.024=0.40x0.08x+0.024=0.40x
x=0.075x=0.075

Answer

Answer

The smoking rate among female policyholders is 7.50%.

7.50%(A)\boxed{7.50\%\quad\text{(A)}}