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.

Input: Consider Input: Problem: an engine moves at the speed of 30 kmph without any coaches attached to it. speed of the train reduces at the rate that varies directly as the square root of the number of coaches attached. when 9 coaches are attached speed decreases to 18 kmph. what will be the speed of train when 16 coaches are attached.
Options: a. 14, b. 16, c. 18, d. 20, e. 22

Output: a

Input: Consider Input: Problem: mary works in a restaurant a maximum of 70 hours. for the first 20 hours, she is paid $ 8 per hour. for each overtime hour, she is paid at a rate which is 25 % higher than her regular rate. how much mary can earn in a week ?
Options: a. 600, b. 620, c. 660, d. 320, e. 400

Output: c

Input: Consider Input: Problem: a train which has 500 m long, is running 100 kmph. in what time will it cross a person moving at 10 kmph in same direction ?
Options: a. 20 sec, b. 25 sec, c. 30 sec, d. 35 sec, e. 40 sec
Output: a