>>4
There exist traveling salesmen. They go from city to city, across the nation, seeking to achieve sales through anal inspection. Traveling from city to city invokes fuel costs, meal costs, and time, and as such, there must be an optimal route for those. The difficultly of choosing the optimality of the route, is what is called the "traveling salesman problem".
There is a minor cost component for anal lube, but since the same number of anuses are inspected, it's a constant across all routes.