Independent solution

How to solve this Conditional Probability question

Setup

Setup

Let F and R denote having at least one filling and at least one root canal. Complement the reported union.

Pr(FcRc)=1Pr(FR)=10.35=0.65\Pr(F^c\cap R^c)=1-\Pr(F\cup R)=1-0.35=0.65

Model

Model

The requested event conditions the joint absence of both procedures on having no fillings.

Pr(RcFc)=Pr(FcRc)Pr(Fc)\Pr(R^c\mid F^c)=\frac{\Pr(F^c\cap R^c)}{\Pr(F^c)}

Compute

Compute

Substitute the joint and marginal probabilities.

Pr(RcFc)=0.650.70=1314=0.928571\Pr(R^c\mid F^c)=\frac{0.65}{0.70}=\frac{13}{14}=0.928571\ldots

Answer

Answer

Given no fillings, the probability of no root canals is approximately 0.93.

0.93(E)\boxed{0.93\quad\text{(E)}}