Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Example Input: Problem: the cross - section of a cannel is a trapezium in shape. if the cannel is 12 m wide at the top and 2 m wide at the bottom and the area of cross - section is 4200 sq m, the depth of cannel is ?
Options: a. 500, b. 600, c. 700, d. 800, e. 900
Example Output: b

Example Input: Problem: the average age of a family of 5 members is 20 years. if the age of the youngest member be 10 years then what was the average age of the family at the time of the birth of the youngest member ?
Options: a. 13.5, b. 14, c. 15, d. 12.5, e. none
Example Output: d

Example Input: 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 $ 160,000 ?
Options: a. 15, b. 16, c. 18, d. 20, e. 24
Example Output:
e