Seven different lecturers are to deliver lectures in seven perday. A, B and C are three oftne lectures. The number of ways in which a routine for theday canr be made such that A delivers his lecture before B and B before C, isA. 420B. 120C. 210D. none of these

Seven different lecturers are to deliver lectures in seven perday. A,

1.	Seven different lecturers are to deliver lectures in seven perday. A, B and C are three oftne lectures. The number of ways in which a routine for theday canr be made such that A delivers his lecture before B and B before C, isA. 420B. 120C. 210D. none of these
Answer» Let us consider the arrangements of 7 objects out of which three are identical and remaining distinct. In each arrangement of these 7 objects, let us mark the first identical item as P, second as Q and third as R. We find that in all the `(7!)/(3!)` arrangements P will occur prior to Q and Q will occur prior to R. In the given sum also the required number of ways `=(7!)/(3!)=840`

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Seven different lecturers are to deliver lectures in seven perday. A, B and C are three oftne lectures. The number of ways in which a routine for theday canr be made such that A delivers his lecture before B and B before C, isA. 420B. 120C. 210D. none of these

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