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: a can finish a work in 60 days, b in 9 days and c in 2 days, b and c start the work but are forced to leave after 3 days. the remaining work was done by a in ?
Options: a. 10 days, b. 25 days, c. 6 days, d. 7 days, e. 8 days
Example Output: b

Example Input: Problem: a sum of money at simple interest amounts to $ 645 in 3 years and to $ 690 in 4 years. the sum is :
Options: a. $ 153, b. $ 510, c. $ 598, d. $ 549, e. $ 675
Example Output: b

Example Input: Problem: if the numerator of a fraction is increased by 20 % and its denominator is diminished by 25 % value of the fraction is 2 / 15. find the original fraction ?
Options: a. 1 / 12, b. 1 / 16, c. 1 / 19, d. 1 / 14, e. 1 / 13
Example Output:
a