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: in a group of cows and chickens, the number of legs was 12 more than twice the number of heads. the number of cows was :
Options: a. 6, b. 7, c. 10, d. 12, e. 14
[EX A]: a

[EX Q]: Problem: the average age of 17 persons in a office is 15 years. out of these, the average age of 5 of them is 14 years and that of the other 9 persons is 16 years. the age of the 15 th person is ?
Options: a. 39, b. 41, c. 45, d. 42, e. 50
[EX A]: b

[EX Q]: Problem: a can do a piece of work in 5 hours; b and c together can do it in 3 hours, while a and c together can do it in 2 hours. how long will b alone take to do it ?
Options: a. 30, b. 20, c. 10, d. 6, e. 5
[EX A]:
a