Brain Teasers & Mental Math
Ten people at a meeting each shake hands exactly once with every other person. How many handshakes?
Model answer
45.
Each handshake is a pair of people: C(10, 2) = 10 × 9 / 2 = 45.
Intuition: each of the 10 people shakes 9 hands = 90, but that counts every handshake twice (once…
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