NDA 2025 · PYQ · Permutations and Combinations · easy
If the number of selections of r as well as (n + r) things from 5n different things are equal, then what is the value of r?
- A.n
- B.2n✓ Correct
- C.3n
- D.4n
C(5n, r) = C(5n, n+r). Using property C(p, q) = C(p, p-q): r + (n+r) = 5n, so 2r = 4n, r = 2n.
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