#33 How many Handshakes

For the first one the answer is 45, because you just add from 9,8,7,6,5,4,3,2,1 and you get 45
so their were 45 handshakes in total.

For the second one is you just add one more person so it makes 55

For the last one you add 11,12,13,14,15,16,17,18,19 and you get the 135.