How many times do 20 students shake hands to greet each other?

one hundred and ninety

In a classmate party, there were 15 students in total. Each student had to shake hands with others once. How many times did he shake hands

The method of permutation and combination is the simplest, 15 * 14 / 2 = 105

How many times did ten students shake hands after meeting

10×（x-1）× 1/2
There are 10 people in total, and each person has to follow a person other than himself, so use 10 × (x-1)
And because there is only one handshake between two people, there will be repeated handshakes, which should be multiplied by 1 / 2

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