A) 6
B) 5
C) 4
D) 7
E) 3
Correct Answer: A
Solution :
\[^{n}{{P}_{r}}={{720}^{n}}{{C}_{r}}\] \[\Rightarrow \] \[\frac{n!}{(n-r)!}=720.\frac{n!}{r!(n-r)!}\] \[\Rightarrow \] \[r!=720=6!\] \[\Rightarrow \] \[r=6\]
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