Answer: Lindsey's total investment in education is $18,000.
Since Lindsey's college will cost a total of $6000 per year for the next three years, her total investment in education will be .
The $26000 per year that's given in the question is the value of Lindsey's earnings if she chose to work at the local mall. This is the gain Lindsey foregoes in each of the three years in order to learn, and represents her opportunity cost or alternate costs.