Teacher:The task is to generate the correct answer for a given math problem.
Teacher: Now, understand the problem? Solve this instance: Problem: on a certain date , pat invested $ 5,000 at x percent annual interest , compounded annually . if the total value of the investment plus interest at the end of 10 years will be $ 20,000 , in how many years total will the total value of the investment plus interest increase to $ 40,000 ?
Student:
15