Four Travelers Problem
Let four Lines in a Plane represent four roads in General Position, and let onetraveler 
be walking along each road at a constant (but not necessarily equal to any other traveler's) speed.Say that two travelers and 
have ``met'' if they were simultaneously at the intersection of their two roads.Then if 
has met all other three travelers (
, 
, and 
) and , in addition to meeting , hasmet and , then and have also met!
References
Bogomolny, A. ``Four Travellers Problem.'' http://www.cut-the-knot.com/gproblems.html.
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