7. There were 80 children at a party. Each child shook hands once with each of the other children. How many handshakes took place?
1st child shakes hand with the other 79 children.
2nd child shakes hand with the other 78 children.
The 1st child has already shaked hand with 2nd child.
79th child shakes hand with the other child (80th) left.
80th child has no more children left to shake hand.
79 + 78 + …… + 1 + 0 = (79 + 0)/2 x 80 = 3160
3160 handshakes took place.