All Pairs Meeting
Spread across a city, there are 4 spy agents. Once in a week, they need to meet up.
However, to avoid suspicion, they can only meet in groups of 3.
How many minimum meetings would they need to make sure that every pair of them has met?
The answer is 3.
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1 solution
Three of them meet, so one is left not meeting anyone, yet.
He and two others meet, so the one left out the first time still has not met the one left out the second time.
One more meeting will remedy that.
That makes three meetings.