Past exam of the year 1 of the computer science course of the University of Oxford / 2018 / Discrete mathematics, continuous mathematics, and probability / 8 / a / ii / Solution

The all-red event $B$ is a subset of the same-colour event $A$. Its probability is $(1/2{)}^3=1/8$, while the three disjoint monochromatic cases giveThus the conditional probability isThe monochromatic outcomes are not equally likely, so counting colours alone would give the wrong conditional answer.