Probability Q1 Thx

(a) no. of hardback =44
no. of paperback=28
Total =44+28 =72
P (the book is paperback) =28/72 =7/18 //

(b)
P(each shelf) = 1/4 (choose equally)
P(the book is hardback) = 1/4 *11/20 +1/4 *8/20 + 1/4*16/20 + 1/4 *9/12
=1/4 * (33/60 + 24/60 + 48/60 + 45/60)
=1/4 *150/60
=1/4 *15/6
=15/24 //

(c)
P(the book chosen is hardback & from shelf C )=1/4 *16/20=4/20 =1/5
P(hardback from shelf C | the book chosen is hardback)
=(1/5) / (15/24)
=8/25 //

2011-11-09 17:19:29 補充：
(b) answer : 15/24 =5/8 //

total no. of hardbook : 44
total no. of paperbook : 28

(a) 28 / (28 + 44) = 28/72 = 7/18

(b) 44 / (28 + 44) = 44/72 = 11/18

(c) 11/18 x 16/44 = 2/9