Task: Question Answering
Input (English): Problem: on a certain date , pat invested $ 10,000 at x percent annual interest , compounded annually . if the total value of the investment plus interest at the end of 12 years will be $ 40,000 , in how many years , the total value of the investment plus interest will increase to $ 120,000 ? 
Options: a ) 15 , b ) 21 , c ) 18 , d ) 20 , e ) 24
Output (English): b