Q. When ten persons shake hands with one another, in how many ways is it possible? (UPSC Prelims 2010)
Answer:
45
Notes: The number of ways ten persons can shake hands with one another is the number of unique handshakes, given by the combination formula C(n, 2) = n(n - 1)/2, where n = 10. This calculates as (10 × 9)/2 = 45. Since handshakes are undirected and no one can shake hands with themselves, each pair shakes hands only once. This is a common combinatorial problem representing edges in a complete graph K₁₀, resulting in 45 unique handshakes.