Task: Question Answering
Input (English): Problem: on a certain date, pat invested $ 7,000 at x percent annual interest, compounded annually. if the total value of the investment plus interest at the end of 10 years will be $ 28,000, in how many years total will the total value of the investment plus interest increase to $ 56,000 ?
Options: a. 15, b. 16, c. 18, d. 20, e. 24
Output (English): a