Final answer:
The monthly parking fee is not relevant to the decision of taking a train or driving for a weekend trip as it is a sunk cost. Decisions should be based on incremental costs and benefits, such as gas prices and train ticket costs.
Step-by-step explanation:
The monthly fee that a student pays to park at school is irrelevant when deciding whether to take a train or drive for a weekend trip to visit an out-of-town friend. The monthly parking fee is a sunk cost, meaning it's a cost that has already occurred and cannot be recovered, and thus should not influence the decision regarding the additional cost of driving versus taking a train for a separate trip. Decisions should be based on the incremental costs and benefits of one choice over another, such as the cost of gasoline, train tickets, and the value of the time spent traveling.
For example, if gas prices are significantly high, a student like Paul might opt to take the train to save on the costs he would incur from driving, regardless of the monthly parking fee he pays. This decision-making process is an important aspect of personal finance and cost management that students will encounter in various scenarios.