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 average age of 15 students of a class is 15 years. out of these, the average age of 8 students is 14 years and that of the other 6 students is 16 years, the age of the 15 th student is
Options: a. 17, b. 12, c. 13, d. 14, e. 15
Example Output: a

Example Input: Problem: a can do a job in 15 days and b in 20 days. if they work on it together for 4 days, then the fraction of the work that is left is ?
Options: a. 2 / 15, b. 8 / 15, c. 3 / 11, d. 1 / 12, e. 6 / 13
Example Output: b

Example Input: Problem: a, b, c and d enter into partnership. a subscribes 1 / 3 of the capital b 1 / 4, c 1 / 5 and d the rest. how much share did a get in a profit of rs. 2400 ?
Options: a. s. 800, b. s. 810, c. s. 820, d. s. 900, e. s. 920
Example Output:
a