A) 8
B) 7
C) 9
D) None of these
Correct Answer: C
Solution :
Since number of derangements in such a problems is given by \[n\ !\left\{ 1-\frac{1}{1\ !}+\frac{1}{2\ !}-\frac{1}{3\ !}+\frac{1}{4\ !}-.......{{(-1)}^{n}}\frac{1}{n\ !} \right\}\] \[\therefore \] Number of derangements are =\[4\ !\left\{ \frac{1}{2\ !}-\frac{1}{3\ !}+\frac{1}{4\ !} \right\}\]\[=12-4+1=9\].
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