Independent solution

How to solve this Conditional Probability question

Setup

Setup

Let f and w be the two policy marginals and let j be their positive intersection probability.

f=P(F),w=P(W),j=P(FW)>0f=P(F),\qquad w=P(W),\qquad j=P(F\cap W)>0

Model

Model

Translate the ratio of the two reverse conditional probabilities. Positivity of j permits cancellation.

jw=2jf\frac{j}{w}=2\frac{j}{f}
f=2wf=2w

Compute

Compute

Express the outside cell using inclusion-exclusion and apply its stated difference from the intersection.

1fw+j=j+0.20f+w=0.801-f-w+j=j+0.20\quad\Longrightarrow\quad f+w=0.80
2w+w=0.80w=415=0.2666672w+w=0.80\quad\Longrightarrow\quad w=\frac{4}{15}=0.266667

Answer

Answer

The flood-policy marginal rounds to 0.27.

0.27(A)\boxed{0.27\quad\text{(A)}}