Final answer:
To recover his $116,000 investment in college education within 4 years, Mark needs to earn a minimum salary of $41,000 annually after graduation.
Step-by-step explanation:
If Mark wants to recover his investment of going to school within 4 years or less after graduating, we need to calculate the total cost of the education plus the opportunity cost of his foregone salary. The total educational investment is the sum of the tuition cost of $68,000 and the lost earnings from his part-time job which amount to $12,000 per year for 4 years, equalling $48,000. Combining these, Mark's total cost is $68,000 + $48,000, which equals $116,000.
To recover this $116,000 within 4 years after graduation, Mark would need to earn at least $29,000 per year on top of his previous earnings to break even. Since Mark was earning $12,000 annually from his part-time job, the minimum salary Mark needs to earn after obtaining his degree to recover his investment in 4 years would therefore be $12,000 + $29,000, which equals $41,000 per year.