Suppose two young people graduate from high school. One start working immediately and earns $1500 per month. The other continues his education for 7 years to train as a physician, costing him $18,000 per year. After graduation, the second student earns $5,000 per month at his new job. How long will it take the second student to make up the cost of his education and surpass the first student financially? After this calculation, explain why considering explicit costs alone is not an accurate representation of the reality of economic costs.