Final answer:
Over the 20-year period, the student invested a total of $48,000. Their portfolio's current value is $128,000. Therefore, they earned $80,000 on their investments.
Step-by-step explanation:
The student is asking how much they earned on their investments over a 20-year period, given that they started investing $200 a month at an average of 8 percent a year and now have an investment portfolio worth $128,000. To solve this problem, we need to calculate the total amount invested over the 20 years, and then subtract this from the current value of the investment.
Firstly, calculate the total amount invested by multiplying $200 (monthly investment) by 12 (months in a year) and then by 20 (number of years invested):
Total amount invested = $200 * 12 * 20 = $48,000.
Now, to find out how much they earned on their investments, subtract the total amount invested from the current value of the investment portfolio:
Earnings on investments = $128,000 (Current value) - $48,000 (Total invested) = $80,000.
Therefore, the correct answer is C. $80,000.