In: Statistics and Probability
Lets rephrase the conditions. There are 18 cards, 6 red ( 2,3,4,5,6,8), 6 purple ( 2,3,4,5,6,8), 6 blue ( 2,3,4,5,6,8). Please clarify in comments if this is not the case.
Assuming that above holds, we need to find the probability that drawn card is 4 given that a blue card is drawn.
By bayes theorem:-
P(Card is 2)=3/18
P(Card is 3)=3/18
P(Card is 4)=3/18
P(Card is 5)=3/18
P(Card is 6)=3/18
P(Card is 8)=3/18
P(Card is Blue | It is 2)=1/3
P(Card is Blue | It is 3)=1/3
P(Card is Blue | It is 4)=1/3
P(Card is Blue | It is 5)=1/3
P(Card is Blue | It is 6)=1/3
P(Card is Blue | It is 8)=1/3
Hence:-