#### Answer

$\frac{1}{3}$

#### Work Step by Step

The probability of event B, assuming that the event A has already occurred, is
called the conditional probability of B, given A. This probability is denoted by
P(B|A).
E: A yellow disk, given that the number selected is at most 3.
A: There are 3 disks given that the number selected is at most 3.
B: There is only 1 way in which a yellow disk is selected from the 3 disks.
P(B|A) = $\frac{1}{3}$