Past exam of the year 1 of the computer science course of the University of Oxford / 2003 / Calculus, probability, linear algebra, discrete mathematics, logic and proof, and models of computation / 5 / a / i / Solution

For events $A,B$ with $\mathbb{P}(B)>0$, the conditional probability isIt measures the probability of $A$ after restricting the sample space to $B$ and renormalizing. This quotient does not define conditional probability given an event of zero probability.