You have a plan to make money by finding a route taking passengers from one destination to another.
Driving between two destinations might cost you money in gas because you have no passenger, while others might make you money because passengers will pay you more than what the gas costs.
Represent money in terms of payout (i.e. +$ are costs and -$ are profit).
Represent this as a graph problem and determine if there are any circuits (i.e. a destination from which you both start and stop) that produces a profit.
If you find this, you can drive this circuit forever and make infinite money!