If the probability that 2 balls drawn (at random without replacement) from a box containing 7 yellow balls, 8 blue balls and 6 red balls are both yellow is 0.1, find the probability that the 2 balls drawn are both yellow when it is known that 1 of them is yellow.

event A = 2 balls are yellow
event B = 1 ball is yellow
event B' = the complement of event B = no ball is yellow

the conditional probability of A given B
= the joint probability of A & B divided by the probability of B
= the joint probability of A & B divided by ( 1 - probability of B' )
=