#### Answer

$\frac{1}{5}$

#### Work Step by Step

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) = $\frac{1}{5}$