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.

[Q]: 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
[A]: b

[Q]: 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
[A]: a

[Q]: Problem: arjun started a business with rs. 20,000 and is joined afterwards by anoop with rs. 30, 000. after how many months did anoop join if the profits at the end of the year are divided equally ?
Options: a. 3, b. 4, c. 5, d. 6, e. 7
[A]:
b