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At a faculty costume party, each teacher from transylvania Middle School came dressed as his or her favorite creepy creature. The teachers arrived one at a time, and when a new teacher came in, each of the teachers already there lined up at the door and shook hands with the newcomer. Count Dracula(Mr.Blab) kept track of the number of hand shkes exchanged. He said that there were 171 handshakes exchanged at the party.
How many teachers came to the costume party

the first teacher did not shake his own hand.

so, lets see how many handsakes Read in two lines,number people,and totalhandshakes.

2 3 4 5 6 7 8 9 10 11 12 13 14
1 3 6 10 15 21 28 36 45 55 66 78 91

15 16 17 18 19
105 120 136 153 171

19

Someone plz tell me the answer

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