Good job all! The opportunity cost is defined as alternative cost: So, you should figure out how much money you could have earned and or saved if you didn't attend the summer school. The answer should be $14,500 = ($6000 lost in wages + $8500 borrowing/dissaving thanks to summer school). Here is another question: What would be the opportunity cost for the same if room and board has to be paid anyway (summer school or no summer school, it doesn't matter)?