Verified Answer :

toporc

There are 20 choices for the first week, 19 for the second week, 18 for the third, 17 for the fourth, 16 for the fifth and 15 for the sixth week, giving a total number of ways of choosing as follows:
\(20\times19\times18\times17\times16\times15=27,907,200\ ways\)

meerkat18

since the book picked for a week is only one and will not be used the following weeks anymore, there is a decrease in 1 in the sample space per week. hence for six weeks, the ways are 20 19 18 17 16*15 equal to 27907200 ways in total

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