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Number of ways of arranging the word "TESTBOOK" such that E always comes between O's.
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O    E   O
The rest of the  letters T,S,T,B and K can come before  first O,Between first O and E, between E and second O and after second O.
For now consider both T as distinct, T1 and T2
Place T1 there are _O_E_O_ 4 places
now a new space is created, for eg if we put T1 at end then _O_E_O_T1_
If somewhere in middle then _O_T1_E_O_
As you can see 5 places for S
Similarly 6 places for T2
7 places for B and 8 places for K
Hence 4*5*6*7*8
Since T1 and T2 are same when interchanged
4*5*6*7*8/2=3360
 

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