Question

If $nC_{r - 1} = 36,\mspace{6mu}^{n}C_{r} = 84$ and $nC_{r + 1} = 126$, then the

value of $r$ is.

• A. 1
• B. 2
• C. 3
• D. None of these

Answer 1

Answer: (C)

3

Answer 2

Here $\frac{nC_{r - 1}}{nC_{r}}$ = $\frac{36}{84}$ and $\frac{nC_{r}}{nC_{r + 1}} = \frac{84}{126}$.

⇒ $3n - 10r = - 3$ and $4n - 10r = 6$

On solving, we get$n = 9$,$r = 3$.