Question:
If ${ }^{n} P_{r}=720$ and ${ }^{n} C_{r}=120$ then find the value of $r$.
Solution:
Given: ${ }^{n} P_{r}=720 \&{ }^{n} C_{r}=120$ Need to find: Value of $r$ We know that, ${ }^{n} P_{r}=r ! X{ }^{n} C_{r}$ Putting the values, $\Rightarrow 720=r ! X 120 \Rightarrow r !=6=3 ! \Rightarrow r=3$.
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