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Answer:
There are 15 guests
Explanation:
If there are n guests then each guest shakes hands with (n−1) other guests. Note, however, that each time a handshake occurs it counts as 2 handshakes (one for each person involved).
Therefore with n guests the number of handshakes will be n×(n−1)2
We are told
XXXn×(n−1)2=105
Therefore
XXXn2−n=210
XXXn2−n−210=0
XXX(n+14)(n−15)=0
Therefore
XXXn=−14 or n=15
And since the number of guest can not be negative:
XXXn=15