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 shop owner professes to sell his articles at certain cost price but he uses false weights with which he cheats by 12 % while buying and by 30 % while selling. what is his percentage profit ?
Options: a. 10.22 %, b. 20.22 %, c. 21.22 %, d. 60 %, e. ca n ' t be calculated
Example Output: d

Example Input: Problem: 5358 x 71 = ?
Options: a. 273762, b. 283758, c. 383298, d. 273258, e. 380418
Example Output: e

Example Input: Problem: a man can row upstream at 25 kmph and downstream at 31 kmph, and then find the speed of the man in still water ?
Options: a. 22, b. 28, c. 30, d. 27, e. 18
Example Output:
b