Thinking Mathematically (6th Edition)

$\frac{1}{5}$
The probability of event B, assuming that event A has already occurred, is called the conditional probability of B, given A. This probability is denoted by P(B|A). E: a 3, given that a red disk is selected. A: There are 5 red disks. B: There is only one red disk with a 3, so we can select a disk numbered 3 in only one way given that it is red. P(B|A) = $\frac{1}{5}$