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taylor was having a party.she invited 20 friends to her party, if each person shook hands with every other person .how many handshakes would there be? please explain,thanks

  • math -

    21 * 20 = ?

  • math -

    Each of the 20 people will shake hands with 19 other people
    20(19)= 380
    but that would include duplicates such as A shaking hands with P, and P shaking hands with A, which is the same handshake
    so number of handshakes = 20(19)/2 = 380/2 = 190

    or

    we are choosing pairs from 20
    = C(20,2) = 190

  • math -

    Thanks, Reiny -- I goofed and didn't consider that we can't count A and P and P and A.

    I included the hostess -- so we have 21 people.

  • math -

    Thanks Ms Sue, and I didn't catch the hostess part, only saw the 20.
    So let's go with your answer รท 2 = 21(20)/2 = 210

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