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: if 50 apprentices can finish a job in 4 hours, and 30 journeymen can finish the same job in 8 hours, how much of the job should be completed by 10 apprentices and 15 journeymen in one hour ?
Options: a. 1 / 9, b. 29 / 180, c. 9 / 80, d. 1 / 5, e. 39 / 121
[A]: c


[Q]: Problem: buses a and b start from a common bus stop x. bus a begins to travel in a straight line away from bus b at a constant rate of 30 miles per hour. one hour later, bus b begins to travel in a straight line in the exact opposite direction at a constant rate of 50 miles per hour. if both buses travel indefinitely, what is the positive difference, in minutes, between the amount of time it takes bus b to cover the exact distance that bus a has covered and the amount of time it takes bus b to cover twice the distance that bus a has covered ?
Options: a. 36, b. 84, c. 132, d. 144, e. 180
[A]: b


[Q]: Problem: a man is 20 years older than his son. in two years, his age will be twice the age of his son. the present age of the son is ?
Options: a. 11, b. 25, c. 27, d. 18, e. 91
[A]:
d