100 people have to shake hands with everyone. How many hand shakes total?

everyone? there 7 billion people, not done before died of old age.

if mean shake hands other 99 people, total number 100 x 99 = 9900, includes duplicates, person 1 shaking 2 , 2 shaking 1.

excluding duplicates, person 1 shakes 99 hands, person 2 shakes 98, person 3, 97, etc. total s = 99 + 98 + 97 + --- + 1
grouping, s = (99+1) + (98+2) + (97+3) + ... + (50+50)
or s = 100+100+100 .... + 100 = 49x100 = 4900

edit, corrected.

100 people have shake hands everyone. how many handshakes total?

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