Can u please explain how you worked out this problem!!!!!!!!!
Imagine that the two book we want to keep together are taped together and treat them as one book. Now there are 9! ways to arrange these "9" books, each way keeping the books together. Now, of those two books, they arranged in 2 ways (BK1:BK2 or BK2:BK1). Hence there are 9!*2 ways to arrange them keeping the two books together and 10! total ways to arrange 10 book.
p = 9!*2/10!
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993