NDA 2025 · PYQ · Permutations and Combinations · easy
How many sides are there in a polygon which has 20 diagonals?
A.6
B.7
C.8✓ Correct
D.10
Explanation
Number of diagonals in a polygon with n sides = n(n-3)/2. Setting n(n-3)/2 = 20: n(n-3) = 40, so n² - 3n - 40 = 0, giving n = (3 + √169)/2 = (3+13)/2 = 8.
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