Graph Theory and Topology - https://en.wikipedia.org/wiki/Seven_Bridges_of_K%C3%B6nigsberg
This has applications in GIS, especially around things like circulation elements city planners use to plan out road placement and design.
Fun fact - Konigsberg no longer exists, it was destroyed by the Russians in WWII and now exists as the Russian exclave of Kaliningrad.
@greylaw89 This is one of my favorite math problems that can be explained to anyone. I can tell it to my college algebra students and they will still understand and be more interested in it than the endless factoring of quadratics.