NDA 2020
- Exam
- NDA
- Year
- 2020
- Subject
- Mathematics
- Chapter
- Permutation and Combination
NDA 2020 · Mathematics · Permutation and Combination
If C(20, n+2)=C(20, n-2) , then what is n equal to?
If , then what is equal to?
Options
- A18
- B25
- C10
- D12
Correct answer
C. 10
Step-by-step solution
For $n=10\begin{aligned}
& (22-10)(21-10)(20-10)(19-10) \\
& =(10+2)(10+1) \cdot 10 \cdot(10-1) \\
& \Rightarrow 12 \cdot 11 \cdot 10 \cdot 9=12 \cdot 11 \cdot 10 \cdot 9
\end{aligned}$$n=10$.