there are 10 people standing equally spaced around a circle. each person knows exactly 3 of the other 9 people then there are 10 possible ways
there are 10 people standing equally spaced around a circle. each person knows exactly 3 of the other 9 people:
There is only $1$ way to do this.
Thus, in total there are $2+5+5+1={(C) }13}$ possible ways.
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