UPSC Prelims 2021·CSAT·Quantitative Aptitude·Combinatorics and Probability

There are 6 persons arranged in a row. Another person has to shake hands with 3 of them so that he should not shake hands with two consecutive persons. In how many distinct possible combinations can the handshakes take place?

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Last updated 23 May 2026, 3:31 pm IST
  1. A3
  2. B4Correct
  3. C5
  4. D6

Explanation

Let the 6 persons be P1, P2, P3, P4, P5, P6. We need to choose 3 persons such that no two chosen persons are consecutive. This is a classic combinatorial problem of selecting 'k' non-consecutive items from 'n' items arranged in a row. The formula for this is C(n - k + 1, k), where C denotes combinations. In this problem: n = 6 (total number of persons) k = 3 (number of persons to be chosen) Applying the formula: Number of combinations = C(6 - 3 + 1, 3) = C(4, 3) Calculating C(4, 3): C(4, 3) = 4! / (3! * (4-3)!) = 4! / (3! * 1!) = (4 * 3 * 2 * 1) / ((3 * 2 * 1) * 1) = 4 The 4 distinct possible combinations are: 1. (P1, P3, P5) 2. (P1, P3, P6) 3. (P1, P4, P6) 4. (P2, P4, P6) Thus, there are 4 distinct possible combinations. The final answer is B
Quantitative Aptitude: There are 6 persons arranged in a row. Another person has to shake hands with 3 of them so that he should not shake hand

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