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.

[EX Q]: Problem: x does a work in 20 days. y does the same work in 40 days. in how many days they together will do the same work ?
Options: a. 10, b. 12, c. 20, d. 30, e. 13.33
[EX A]: e

[EX Q]: Problem: a man sitting in a train which is traveling at 36 kmph observes that a goods train, traveling in opposite direction, takes 10 seconds to pass him. if the goods train is 240 m long, find its speed
Options: a. 50 kmph, b. 56 kmph, c. 58 kmph, d. 62 kmph, e. 72 kmph
[EX A]: a

[EX Q]: Problem: a circular swimming pool is surrounded by a concrete wall 4 feet wide. if the area of the wall is 11 / 25 of the area of the pool, then the radius of the pool in feet is ?
Options: ['a. 22', 'b. 20', 'c. 9', 'd. 8', 'e. 1']
[EX A]:
b