According to my calculations you are correct and not the textbook.
Exercise 3d Tree Diagrams, Section B, Question 2. (Page 201)
A box contains 20 chocolates, of which 15 have Soft centres and 5 have Hard centres. Two chocolates are taken at random, one after the other.
Calculate the probability that
(a) both chocolates have Soft centres, [BOOK ANSWER: ]
(b) one of each sort of chocolates is taken, [BOOK ANSWER: ]
(c) both chocolates have Hard centres, given that the second chocolates has a Hard centres. [BOOK ANSWER: ]
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I manage as shown below, to successfully obtain the first two answers correct, but question (c) don't corresponds to any solutions that I could ever think of.
My Solution:
(a)
MATCHED
(b)
MATCHED
(c) I need Help here, this conditional probability is ok I hope:
MISMATCHED !!!
Is it a mistake of interpretation from my part or from the Book? Your Help is welcomed, Thank you
I think you are right too.
I just said one dark choc doesn't count-it's already accounted for. Therefore the probability of the first one being a dark chocolate is 4/19
Mind you, I'd believe Plato way before I'd believe me. I am here to ask another probability question.