2 cases where the ball is a new one
4 new 4 old -> 1/2
3 cases where it's an old one
3 new 5 old -> 3/8
5 cases total
2/5*1/2 + 3/5*3/8 = 17/40
too tired for the second one sorry
I need these solutions in 3 hours.
I've 2 boxes. THe first box has 2 new and 3 old balls. The second box has 3 new and 4 old balls. I took one ball from the first box and put it into the second box. Now I take the ball from the second box.
What's the probability that:
1) The ball taken from the second box is new.
2) The ball taken from the first box was new if I know that the ball taken from the second box was old.
It seems to me that one of these is the old ball (23/40) taken and the second one of these is the new ball (17/40) taken? How to make difference?