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There are 10 persons named P_1,P_2,P_3, ... P_{10}. Out of 10 persons, 5 persons are to be arranged in a line such that in each arrangement P1 must occur whereas P4 and P5 do not occur. Find the number of such possible arrangements. [Hint: Required number of arrangement =7C_4\times 5!]

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Given that  P_1,P_2,P_3, ... P_{10} 

persons out of which 5 are to be arranged but P_1must occur

whereas P4and P5 never occur.  So, now we only have to select 4 out of 7 persons

Number of selections=^7C_4=35

Number of the arrangement of 5 persons=35*5!=35*120=4200

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