UPSC Prelims 2024·CSAT·Quantitative Aptitude·Combinatorics and Probability

ABCD is a square. One point on each of AB and CD; and two distinct points on each of BC and DA are chosen. How many distinct triangles can be drawn using any three points as vertices out of these six points?

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Last updated 23 May 2026, 3:31 pm IST
  1. A16
  2. B18
  3. C20Correct
  4. D24

Explanation

To find the number of distinct triangles, we use the combination formula to select 3 points out of the total 6 points available. 1. Identify the total points: There is 1 point on AB, 1 point on CD, 2 points on BC, and 2 points on DA. Total points = 1 + 1 + 2 + 2 = 6 points. 2. Calculate total combinations: The number of ways to choose any 3 points out of 6 is calculated as (6 x 5 x 4) / (3 x 2 x 1) = 20. 3. Check for collinear points: A triangle cannot be formed if all 3 chosen points lie on the same straight line (collinear). In this square, each side contains at most 2 points. Since no side has 3 points, it is impossible to pick 3 points that are collinear. Therefore, every combination of 3 points forms a valid triangle. The total number of distinct triangles is 20. The correct option is C.
Quantitative Aptitude: ABCD is a square. One point on each of AB and CD; and two distinct points on each of BC and DA are chosen. How many dist

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