See example 3 at

https://math.berkeley.edu/~ehallman/summer-2015/problemSheet6-2-sols.pdf

Why the answer is 105/221 and not

(3/8)(40/100)/(3/8(40/100)+2/9(30/100)+(6/7)(30/100))=63/199?

Stupid question of mine, it says that A BLACK ball is drawn, not a red as in example 1 and 2. 

Edited October 25, 20214 yr by Theodoros Kiriakopoulos

21 hours ago, Theodoros Kiriakopoulos said:

Stupid question of mine, it says that A BLACK ball is drawn, not a red as in example 1 and 2. 

!

Moderator Note

Are you saying you've figured this out and no longer need any help, or should I leave this open?

 

I've figured this out.