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NDA (Held On: 21 Apr 2019) Maths Previous Year paper

Option 2 : 10

Group X 2021 Full Mock Test

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__Concept:__

The notation C(n, r) is the number of combinations/groups of n different things taking r at a time and is given by: \(C\;\left( {n,\;r} \right) = \frac{{{\rm{n}}!}}{{{\rm{r}}!\left( {{\rm{n\;}} - {\rm{\;r}}} \right)!}}\)

If C(n, x) = C(n, y), then x + y = n.

** Calculation**:

Given C(20, n + 2) = C(20, n – 2)

As we know that, if C(n, x) = C(n, y), then x + y = n.

⇒ (n + 2) + (n – 2) = 20

⇒ 2n = 20

⇒ n = 10