Let four lines in a plane represent four roads in general position, and let one traveler
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 
, has met 
and 
, then 
and 
have also met!
Four Travelers Problem
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References
Bogomolny, A. "Four Travellers Problem." http://www.cut-the-knot.org/gproblems.shtml.Referenced on Wolfram|Alpha
Four Travelers ProblemCite this as:
Weisstein, Eric W. "Four Travelers Problem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/FourTravelersProblem.html