how many handshakes

20 students 

1=20

1=19

39

1=18

1=17

 35

 74

 1=16

1=15

31

1=14

1=13

1=12

39

 70

144

1=11

1=10

1=9

1=8

1=7

1=6

1=5

1=4

1=3

1=2

1=1 

66

144+66=210

the answer is 210