At a party 66 handshakes occurred. Each person shook hands

Lucian Maddox

Lucian Maddox

Answered question

2022-07-07

At a party 66 handshakes occurred. Each person shook hands exactly once with each of the other people present. How many people were present?

Answer & Explanation

Yair Boyle

Yair Boyle

Beginner2022-07-08Added 10 answers

Step 1
Let's start with small numbers of people and handshakes and move from there. I'll represent people with letters to show the handshakes:
If we have 2 people, there is 1 handshake (AB).
If we have 3 people, there are 3 handshakes (AB, AC, BC).
If we have 4 people, there are 6 handshakes (AB, AC, AD, BC, BD, CD).
If we have 5 people, there are 10 handshakes (AB, AC, AD, AE, BC, BD, BE, CD, CE, DE).
Step 2
See that we can express the number of handshakes as the sum of consecutive positive integers, starting with 1, i.e. 1 + 2 + 3 + ... + ( n - 1 ) and the number of people present is n
Let's test this with 5 people. We have 1 + 2 + 3 + 4 = 10 handshakes. n - 1 = 4 n = 5 which is the number of people.
So what we need to do is add up to 66 and we'll be able to find the number of people:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 = 66
n - 1 = 11 n = 12

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