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NDA 2013
Exam
NDA
Year
2013
Subject
Mathematics
Chapter
Permutation and Combination

NDA 2013 · Mathematics · Permutation and Combination

If C(28,2 r)=C(28,2 r-4) , then what is r equal to?

If C(28,2r)=C(28,2r−4)\mathrm{C}(28,2 r)=\mathrm{C}(28,2 r-4), then what is rr equal to?

Options

  1. A7
  2. B8
  3. C12
  4. D16

Correct answer

B. 8

Step-by-step solution

28C2r=28C2r−4⇒2r+2r−4=28⇒4r=32r=8\begin{aligned} & { }^{28} \mathrm{C}_{2 r}={ }^{28} \mathrm{C}_{2 r-4} \\ & \Rightarrow 2 r+2 r-4=28 \\ & \Rightarrow 4 r=32 \\ & r=8 \end{aligned}
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