Consider that the probability of an event is given by,
[tex]\text{Probability of an event}=\frac{\text{ No. of events favorable to the event}}{\text{ Total possible no. of outcomes}}[/tex]Consider the following events,
A = event that a voter plans to vote 'yes' on issue P
B = event that a voter plans to vote 'no' on issue Q
Then, given that event A has already occurred, the probability of occurrence of B, is given by,
[tex]P(\frac{B}{A})=\frac{P(A\cap B)}{P(A)}[/tex]