#33 How Many Handshakes

1) If there were 10 student in the class there would be 45 handshakes because I added 9,8,7,6 and I kept going down until 1.

2) If the 11th student walks in there would be 55 handshakes because everybody shook hands and the 11th need to shakes hands with 10 people. 

3) If there were 20 students in the classroom there would be 190