Q. $S_{1},\ S_{2},\ldots,S_{10}$ are the speakers in a conference. If $S_{1}$ addresses only after $S_{2}$, then the number of ways the speakers address is
A) $10!$
B) $9!$
C) $10 \times 8!$
D) $\frac{10!}{2!}$
Explanation
(d)
Since, $S_{1}$ speak after $S_{2}$, Therefore two places can be chosen out of 10 place in $\ ^{10}C_{2}$ ways and rest of the 8 speakers can speak in $8!$ ways.
$\therefore$ Required number of ways$= \ ^{10}C_{2}.8!$
$= \frac{10!}{2!8!}.8! = \frac{10!}{2!}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.