400/5=80 so 80 books per book shelf.
Well, the problem is that he does not replace the first marble after he draws it. Therefore, yes, he has a 2/12 probability of drawing a yellow marble first, but then he has a 4/11 probability of drawing a red marble second, not 4/12. This is because there are going to only be 11 marbles left in the bag after he draws the first marble and does not replace it.
The correct answer would be:
(2/12)(4/11)=8/132=4/66=2/33
I’m not sure but u need more information on your question
