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: a can do a piece of work in 4 hours; b and c together can do it in 3 hours, while a and c together can do it 2 hours. how long will b alone take to do it ?
Options: a. 8 hours, b. 10 hours, c. 12 hours, d. 24 hours, e. none of these
[EX A]: c

[EX Q]: Problem: if a / b = 1 / 3, b / c = 2, c / d = 1 / 2, d / e = 3 and e / f = 1 / 10, then what is the value of abc / def ?
Options: a. 27 / 4, b. 27 / 8, c. 3 / 4, d. 3 / 20, e. 1 / 4
[EX A]: d

[EX Q]: Problem: a bag has 9 green balls and 23 pink balls. how many green balls should be added to the bag so that the ratio of green balls and pink balls in the bag becomes 1: 1 ?
Options: a. 10, b. 11, c. 12, d. 13, e. 14
[EX A]:
e