Question

Seven different lecturers are to deliver lectures in seven periods of a class on a particular day. A, B, C are the three lecturers. The number of ways in which the routine for the day can be made such that A delivers his lecture before B and B before C is-
a) 420
b) 120
c) 210
d) 840

Answer 1

Dear student
$$\begin{gathered} AsA,BandCaretobetreatedidenticalinarrangement,hencerequirednumberofwaysis \\ \frac{7!}{3!}=\frac{7\times 6\times 5\times 4\times 3!}{3!}=840ways \end{gathered}$$
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