there is only one way to arrange all the eight
relative to each other because they are not distinct. we have
![\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }](/tex.php?f=\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ }b\underline{\ \ })
now there are nine places to place an
(and not more than one
in one place). because we have four
symbols, we need to choose four of these nine places to insert an
. order does not matter because the
are not distinct.
thus there are 1 · c(9,4) ways, or, if we calculate it,
![1\cdot c(9,4) = \dfrac{9! }{4! (9-4)! } = \dfrac{9\cdot8\cdot7\cdot6\cdot5! }{4! 5! } = \dfrac{9\cdot8\cdot7\cdot6}{4\cdot3\cdot2\cdot1} = 126.](/tex.php?f=1\cdot c(9,4) = \dfrac{9! }{4! (9-4)! } = \dfrac{9\cdot8\cdot7\cdot6\cdot5! }{4! 5! } = \dfrac{9\cdot8\cdot7\cdot6}{4\cdot3\cdot2\cdot1} = 126.)
thus there are 126 ways to form such "words."